[java] support required nullable properties on java okhttp-gson generator (#15462)

* Support required nullable properties on java okhttp-gson generator

* regen client after merge

* covering cases
* nullable - true ; required - true
* nullable - false ; required - true
* nullable - true ; required - false
* nullable - false ; required - false

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/AndroidManifest.xml

@@ -0,0 +1,3 @@
+<manifest package="org.openapitools.client" xmlns:android="http://schemas.android.com/apk/res/android">
+    <application />
+</manifest>

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/ApiCallback.java

@@ -0,0 +1,62 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+import java.io.IOException;
+
+import java.util.Map;
+import java.util.List;
+
+/**
+ * Callback for asynchronous API call.
+ *
+ * @param <T> The return type
+ */
+public interface ApiCallback<T> {
+    /**
+     * This is called when the API call fails.
+     *
+     * @param e The exception causing the failure
+     * @param statusCode Status code of the response if available, otherwise it would be 0
+     * @param responseHeaders Headers of the response if available, otherwise it would be null
+     */
+    void onFailure(ApiException e, int statusCode, Map<String, List<String>> responseHeaders);
+
+    /**
+     * This is called when the API call succeeded.
+     *
+     * @param result The result deserialized from response
+     * @param statusCode Status code of the response
+     * @param responseHeaders Headers of the response
+     */
+    void onSuccess(T result, int statusCode, Map<String, List<String>> responseHeaders);
+
+    /**
+     * This is called when the API upload processing.
+     *
+     * @param bytesWritten bytes Written
+     * @param contentLength content length of request body
+     * @param done write end
+     */
+    void onUploadProgress(long bytesWritten, long contentLength, boolean done);
+
+    /**
+     * This is called when the API download processing.
+     *
+     * @param bytesRead bytes Read
+     * @param contentLength content length of the response
+     * @param done Read end
+     */
+    void onDownloadProgress(long bytesRead, long contentLength, boolean done);
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/ApiException.java

@@ -0,0 +1,166 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+import java.util.Map;
+import java.util.List;
+
+import javax.ws.rs.core.GenericType;
+
+/**
+ * <p>ApiException class.</p>
+ */
+@SuppressWarnings("serial")
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class ApiException extends Exception {
+    private int code = 0;
+    private Map<String, List<String>> responseHeaders = null;
+    private String responseBody = null;
+ 
+    /**
+     * <p>Constructor for ApiException.</p>
+     */
+    public ApiException() {}
+
+    /**
+     * <p>Constructor for ApiException.</p>
+     *
+     * @param throwable a {@link java.lang.Throwable} object
+     */
+    public ApiException(Throwable throwable) {
+        super(throwable);
+    }
+
+    /**
+     * <p>Constructor for ApiException.</p>
+     *
+     * @param message the error message
+     */
+    public ApiException(String message) {
+        super(message);
+    }
+
+    /**
+     * <p>Constructor for ApiException.</p>
+     *
+     * @param message the error message
+     * @param throwable a {@link java.lang.Throwable} object
+     * @param code HTTP status code
+     * @param responseHeaders a {@link java.util.Map} of HTTP response headers
+     * @param responseBody the response body
+     */
+    public ApiException(String message, Throwable throwable, int code, Map<String, List<String>> responseHeaders, String responseBody) {
+        super(message, throwable);
+        this.code = code;
+        this.responseHeaders = responseHeaders;
+        this.responseBody = responseBody;
+    }
+
+    /**
+     * <p>Constructor for ApiException.</p>
+     *
+     * @param message the error message
+     * @param code HTTP status code
+     * @param responseHeaders a {@link java.util.Map} of HTTP response headers
+     * @param responseBody the response body
+     */
+    public ApiException(String message, int code, Map<String, List<String>> responseHeaders, String responseBody) {
+        this(message, (Throwable) null, code, responseHeaders, responseBody);
+    }
+
+    /**
+     * <p>Constructor for ApiException.</p>
+     *
+     * @param message the error message
+     * @param throwable a {@link java.lang.Throwable} object
+     * @param code HTTP status code
+     * @param responseHeaders a {@link java.util.Map} of HTTP response headers
+     */
+    public ApiException(String message, Throwable throwable, int code, Map<String, List<String>> responseHeaders) {
+        this(message, throwable, code, responseHeaders, null);
+    }
+
+    /**
+     * <p>Constructor for ApiException.</p>
+     *
+     * @param code HTTP status code
+     * @param responseHeaders a {@link java.util.Map} of HTTP response headers
+     * @param responseBody the response body
+     */
+    public ApiException(int code, Map<String, List<String>> responseHeaders, String responseBody) {
+        this("Response Code: " + code + " Response Body: " + responseBody, (Throwable) null, code, responseHeaders, responseBody);
+    }
+
+    /**
+     * <p>Constructor for ApiException.</p>
+     *
+     * @param code HTTP status code
+     * @param message a {@link java.lang.String} object
+     */
+    public ApiException(int code, String message) {
+        super(message);
+        this.code = code;
+    }
+
+    /**
+     * <p>Constructor for ApiException.</p>
+     *
+     * @param code HTTP status code
+     * @param message the error message
+     * @param responseHeaders a {@link java.util.Map} of HTTP response headers
+     * @param responseBody the response body
+     */
+    public ApiException(int code, String message, Map<String, List<String>> responseHeaders, String responseBody) {
+        this(code, message);
+        this.responseHeaders = responseHeaders;
+        this.responseBody = responseBody;
+    }
+
+    /**
+     * Get the HTTP status code.
+     *
+     * @return HTTP status code
+     */
+    public int getCode() {
+        return code;
+    }
+
+    /**
+     * Get the HTTP response headers.
+     *
+     * @return A map of list of string
+     */
+    public Map<String, List<String>> getResponseHeaders() {
+        return responseHeaders;
+    }
+
+    /**
+     * Get the HTTP response body.
+     *
+     * @return Response body in the form of string
+     */
+    public String getResponseBody() {
+        return responseBody;
+    }
+
+    /**
+     * Get the exception message including HTTP response data.
+     *
+     * @return The exception message
+     */
+    public String getMessage() {
+        return String.format("Message: %s%nHTTP response code: %s%nHTTP response body: %s%nHTTP response headers: %s",
+                super.getMessage(), this.getCode(), this.getResponseBody(), this.getResponseHeaders());
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/ApiResponse.java

@@ -0,0 +1,76 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * API response returned by API call.
+ */
+public class ApiResponse<T> {
+    final private int statusCode;
+    final private Map<String, List<String>> headers;
+    final private T data;
+
+    /**
+     * <p>Constructor for ApiResponse.</p>
+     *
+     * @param statusCode The status code of HTTP response
+     * @param headers The headers of HTTP response
+     */
+    public ApiResponse(int statusCode, Map<String, List<String>> headers) {
+        this(statusCode, headers, null);
+    }
+
+    /**
+     * <p>Constructor for ApiResponse.</p>
+     *
+     * @param statusCode The status code of HTTP response
+     * @param headers The headers of HTTP response
+     * @param data The object deserialized from response bod
+     */
+    public ApiResponse(int statusCode, Map<String, List<String>> headers, T data) {
+        this.statusCode = statusCode;
+        this.headers = headers;
+        this.data = data;
+    }
+
+    /**
+     * <p>Get the <code>status code</code>.</p>
+     *
+     * @return the status code
+     */
+    public int getStatusCode() {
+        return statusCode;
+    }
+
+    /**
+     * <p>Get the <code>headers</code>.</p>
+     *
+     * @return a {@link java.util.Map} of headers 
+     */
+    public Map<String, List<String>> getHeaders() {
+        return headers;
+    }
+
+    /**
+     * <p>Get the <code>data</code>.</p>
+     *
+     * @return the data
+     */
+    public T getData() {
+        return data;
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/Configuration.java

@@ -0,0 +1,41 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class Configuration {
+    public static final String VERSION = "1.0.0";
+
+    private static ApiClient defaultApiClient = new ApiClient();
+
+    /**
+     * Get the default API client, which would be used when creating API
+     * instances without providing an API client.
+     *
+     * @return Default API client
+     */
+    public static ApiClient getDefaultApiClient() {
+        return defaultApiClient;
+    }
+
+    /**
+     * Set the default API client, which would be used when creating API
+     * instances without providing an API client.
+     *
+     * @param apiClient API client
+     */
+    public static void setDefaultApiClient(ApiClient apiClient) {
+        defaultApiClient = apiClient;
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/GzipRequestInterceptor.java

@@ -0,0 +1,85 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+import okhttp3.*;
+import okio.Buffer;
+import okio.BufferedSink;
+import okio.GzipSink;
+import okio.Okio;
+
+import java.io.IOException;
+
+/**
+ * Encodes request bodies using gzip.
+ *
+ * Taken from https://github.com/square/okhttp/issues/350
+ */
+class GzipRequestInterceptor implements Interceptor {
+    @Override
+    public Response intercept(Chain chain) throws IOException {
+        Request originalRequest = chain.request();
+        if (originalRequest.body() == null || originalRequest.header("Content-Encoding") != null) {
+            return chain.proceed(originalRequest);
+        }
+
+        Request compressedRequest = originalRequest.newBuilder()
+                                                   .header("Content-Encoding", "gzip")
+                                                   .method(originalRequest.method(), forceContentLength(gzip(originalRequest.body())))
+                                                   .build();
+        return chain.proceed(compressedRequest);
+    }
+
+    private RequestBody forceContentLength(final RequestBody requestBody) throws IOException {
+        final Buffer buffer = new Buffer();
+        requestBody.writeTo(buffer);
+        return new RequestBody() {
+            @Override
+            public MediaType contentType() {
+                return requestBody.contentType();
+            }
+
+            @Override
+            public long contentLength() {
+                return buffer.size();
+            }
+
+            @Override
+            public void writeTo(BufferedSink sink) throws IOException {
+                sink.write(buffer.snapshot());
+            }
+        };
+    }
+
+    private RequestBody gzip(final RequestBody body) {
+        return new RequestBody() {
+            @Override
+            public MediaType contentType() {
+                return body.contentType();
+            }
+
+            @Override
+            public long contentLength() {
+                return -1; // We don't know the compressed length in advance!
+            }
+
+            @Override
+            public void writeTo(BufferedSink sink) throws IOException {
+                BufferedSink gzipSink = Okio.buffer(new GzipSink(sink));
+                body.writeTo(gzipSink);
+                gzipSink.close();
+            }
+        };
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/JSON.java

@@ -0,0 +1,408 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapter;
+import com.google.gson.internal.bind.util.ISO8601Utils;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import com.google.gson.JsonElement;
+import io.gsonfire.GsonFireBuilder;
+import io.gsonfire.TypeSelector;
+
+import okio.ByteString;
+
+import java.io.IOException;
+import java.io.StringReader;
+import java.lang.reflect.Type;
+import java.text.DateFormat;
+import java.text.ParseException;
+import java.text.ParsePosition;
+import java.time.LocalDate;
+import java.time.OffsetDateTime;
+import java.time.format.DateTimeFormatter;
+import java.util.Date;
+import java.util.Locale;
+import java.util.Map;
+import java.util.HashMap;
+
+/*
+ * A JSON utility class
+ *
+ * NOTE: in the future, this class may be converted to static, which may break
+ *       backward-compatibility
+ */
+public class JSON {
+    private static Gson gson;
+    private static boolean isLenientOnJson = false;
+    private static DateTypeAdapter dateTypeAdapter = new DateTypeAdapter();
+    private static SqlDateTypeAdapter sqlDateTypeAdapter = new SqlDateTypeAdapter();
+    private static OffsetDateTimeTypeAdapter offsetDateTimeTypeAdapter = new OffsetDateTimeTypeAdapter();
+    private static LocalDateTypeAdapter localDateTypeAdapter = new LocalDateTypeAdapter();
+    private static ByteArrayAdapter byteArrayAdapter = new ByteArrayAdapter();
+
+    @SuppressWarnings("unchecked")
+    public static GsonBuilder createGson() {
+        GsonFireBuilder fireBuilder = new GsonFireBuilder()
+        ;
+        GsonBuilder builder = fireBuilder.createGsonBuilder();
+        return builder;
+    }
+
+    private static String getDiscriminatorValue(JsonElement readElement, String discriminatorField) {
+        JsonElement element = readElement.getAsJsonObject().get(discriminatorField);
+        if (null == element) {
+            throw new IllegalArgumentException("missing discriminator field: <" + discriminatorField + ">");
+        }
+        return element.getAsString();
+    }
+
+    /**
+     * Returns the Java class that implements the OpenAPI schema for the specified discriminator value.
+     *
+     * @param classByDiscriminatorValue The map of discriminator values to Java classes.
+     * @param discriminatorValue The value of the OpenAPI discriminator in the input data.
+     * @return The Java class that implements the OpenAPI schema
+     */
+    private static Class getClassByDiscriminator(Map classByDiscriminatorValue, String discriminatorValue) {
+        Class clazz = (Class) classByDiscriminatorValue.get(discriminatorValue);
+        if (null == clazz) {
+            throw new IllegalArgumentException("cannot determine model class of name: <" + discriminatorValue + ">");
+        }
+        return clazz;
+    }
+
+    {
+        GsonBuilder gsonBuilder = createGson();
+        gsonBuilder.registerTypeAdapter(Date.class, dateTypeAdapter);
+        gsonBuilder.registerTypeAdapter(java.sql.Date.class, sqlDateTypeAdapter);
+        gsonBuilder.registerTypeAdapter(OffsetDateTime.class, offsetDateTimeTypeAdapter);
+        gsonBuilder.registerTypeAdapter(LocalDate.class, localDateTypeAdapter);
+        gsonBuilder.registerTypeAdapter(byte[].class, byteArrayAdapter);
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Category.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.ModelApiResponse.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Order.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Pet.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.PetWithRequiredNullableCases1.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.PetWithRequiredNullableCases2.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Tag.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.User.CustomTypeAdapterFactory());
+        gson = gsonBuilder.create();
+    }
+
+    /**
+     * Get Gson.
+     *
+     * @return Gson
+     */
+    public static Gson getGson() {
+        return gson;
+    }
+
+    /**
+     * Set Gson.
+     *
+     * @param gson Gson
+     */
+    public static void setGson(Gson gson) {
+        JSON.gson = gson;
+    }
+
+    public static void setLenientOnJson(boolean lenientOnJson) {
+        isLenientOnJson = lenientOnJson;
+    }
+
+    /**
+     * Serialize the given Java object into JSON string.
+     *
+     * @param obj Object
+     * @return String representation of the JSON
+     */
+    public static String serialize(Object obj) {
+        return gson.toJson(obj);
+    }
+
+    /**
+     * Deserialize the given JSON string to Java object.
+     *
+     * @param <T>        Type
+     * @param body       The JSON string
+     * @param returnType The type to deserialize into
+     * @return The deserialized Java object
+     */
+    @SuppressWarnings("unchecked")
+    public static <T> T deserialize(String body, Type returnType) {
+        try {
+            if (isLenientOnJson) {
+                JsonReader jsonReader = new JsonReader(new StringReader(body));
+                // see https://google-gson.googlecode.com/svn/trunk/gson/docs/javadocs/com/google/gson/stream/JsonReader.html#setLenient(boolean)
+                jsonReader.setLenient(true);
+                return gson.fromJson(jsonReader, returnType);
+            } else {
+                return gson.fromJson(body, returnType);
+            }
+        } catch (JsonParseException e) {
+            // Fallback processing when failed to parse JSON form response body:
+            // return the response body string directly for the String return type;
+            if (returnType.equals(String.class)) {
+                return (T) body;
+            } else {
+                throw (e);
+            }
+        }
+    }
+
+    /**
+     * Gson TypeAdapter for Byte Array type
+     */
+    public static class ByteArrayAdapter extends TypeAdapter<byte[]> {
+
+        @Override
+        public void write(JsonWriter out, byte[] value) throws IOException {
+            if (value == null) {
+                out.nullValue();
+            } else {
+                out.value(ByteString.of(value).base64());
+            }
+        }
+
+        @Override
+        public byte[] read(JsonReader in) throws IOException {
+            switch (in.peek()) {
+                case NULL:
+                    in.nextNull();
+                    return null;
+                default:
+                    String bytesAsBase64 = in.nextString();
+                    ByteString byteString = ByteString.decodeBase64(bytesAsBase64);
+                    return byteString.toByteArray();
+            }
+        }
+    }
+
+    /**
+     * Gson TypeAdapter for JSR310 OffsetDateTime type
+     */
+    public static class OffsetDateTimeTypeAdapter extends TypeAdapter<OffsetDateTime> {
+
+        private DateTimeFormatter formatter;
+
+        public OffsetDateTimeTypeAdapter() {
+            this(DateTimeFormatter.ISO_OFFSET_DATE_TIME);
+        }
+
+        public OffsetDateTimeTypeAdapter(DateTimeFormatter formatter) {
+            this.formatter = formatter;
+        }
+
+        public void setFormat(DateTimeFormatter dateFormat) {
+            this.formatter = dateFormat;
+        }
+
+        @Override
+        public void write(JsonWriter out, OffsetDateTime date) throws IOException {
+            if (date == null) {
+                out.nullValue();
+            } else {
+                out.value(formatter.format(date));
+            }
+        }
+
+        @Override
+        public OffsetDateTime read(JsonReader in) throws IOException {
+            switch (in.peek()) {
+                case NULL:
+                    in.nextNull();
+                    return null;
+                default:
+                    String date = in.nextString();
+                    if (date.endsWith("+0000")) {
+                        date = date.substring(0, date.length()-5) + "Z";
+                    }
+                    return OffsetDateTime.parse(date, formatter);
+            }
+        }
+    }
+
+    /**
+     * Gson TypeAdapter for JSR310 LocalDate type
+     */
+    public static class LocalDateTypeAdapter extends TypeAdapter<LocalDate> {
+
+        private DateTimeFormatter formatter;
+
+        public LocalDateTypeAdapter() {
+            this(DateTimeFormatter.ISO_LOCAL_DATE);
+        }
+
+        public LocalDateTypeAdapter(DateTimeFormatter formatter) {
+            this.formatter = formatter;
+        }
+
+        public void setFormat(DateTimeFormatter dateFormat) {
+            this.formatter = dateFormat;
+        }
+
+        @Override
+        public void write(JsonWriter out, LocalDate date) throws IOException {
+            if (date == null) {
+                out.nullValue();
+            } else {
+                out.value(formatter.format(date));
+            }
+        }
+
+        @Override
+        public LocalDate read(JsonReader in) throws IOException {
+            switch (in.peek()) {
+                case NULL:
+                    in.nextNull();
+                    return null;
+                default:
+                    String date = in.nextString();
+                    return LocalDate.parse(date, formatter);
+            }
+        }
+    }
+
+    public static void setOffsetDateTimeFormat(DateTimeFormatter dateFormat) {
+        offsetDateTimeTypeAdapter.setFormat(dateFormat);
+    }
+
+    public static void setLocalDateFormat(DateTimeFormatter dateFormat) {
+        localDateTypeAdapter.setFormat(dateFormat);
+    }
+
+    /**
+     * Gson TypeAdapter for java.sql.Date type
+     * If the dateFormat is null, a simple "yyyy-MM-dd" format will be used
+     * (more efficient than SimpleDateFormat).
+     */
+    public static class SqlDateTypeAdapter extends TypeAdapter<java.sql.Date> {
+
+        private DateFormat dateFormat;
+
+        public SqlDateTypeAdapter() {}
+
+        public SqlDateTypeAdapter(DateFormat dateFormat) {
+            this.dateFormat = dateFormat;
+        }
+
+        public void setFormat(DateFormat dateFormat) {
+            this.dateFormat = dateFormat;
+        }
+
+        @Override
+        public void write(JsonWriter out, java.sql.Date date) throws IOException {
+            if (date == null) {
+                out.nullValue();
+            } else {
+                String value;
+                if (dateFormat != null) {
+                    value = dateFormat.format(date);
+                } else {
+                    value = date.toString();
+                }
+                out.value(value);
+            }
+        }
+
+        @Override
+        public java.sql.Date read(JsonReader in) throws IOException {
+            switch (in.peek()) {
+                case NULL:
+                    in.nextNull();
+                    return null;
+                default:
+                    String date = in.nextString();
+                    try {
+                        if (dateFormat != null) {
+                            return new java.sql.Date(dateFormat.parse(date).getTime());
+                        }
+                        return new java.sql.Date(ISO8601Utils.parse(date, new ParsePosition(0)).getTime());
+                    } catch (ParseException e) {
+                        throw new JsonParseException(e);
+                    }
+            }
+        }
+    }
+
+    /**
+     * Gson TypeAdapter for java.util.Date type
+     * If the dateFormat is null, ISO8601Utils will be used.
+     */
+    public static class DateTypeAdapter extends TypeAdapter<Date> {
+
+        private DateFormat dateFormat;
+
+        public DateTypeAdapter() {}
+
+        public DateTypeAdapter(DateFormat dateFormat) {
+            this.dateFormat = dateFormat;
+        }
+
+        public void setFormat(DateFormat dateFormat) {
+            this.dateFormat = dateFormat;
+        }
+
+        @Override
+        public void write(JsonWriter out, Date date) throws IOException {
+            if (date == null) {
+                out.nullValue();
+            } else {
+                String value;
+                if (dateFormat != null) {
+                    value = dateFormat.format(date);
+                } else {
+                    value = ISO8601Utils.format(date, true);
+                }
+                out.value(value);
+            }
+        }
+
+        @Override
+        public Date read(JsonReader in) throws IOException {
+            try {
+                switch (in.peek()) {
+                    case NULL:
+                        in.nextNull();
+                        return null;
+                    default:
+                        String date = in.nextString();
+                        try {
+                            if (dateFormat != null) {
+                                return dateFormat.parse(date);
+                            }
+                            return ISO8601Utils.parse(date, new ParsePosition(0));
+                        } catch (ParseException e) {
+                            throw new JsonParseException(e);
+                        }
+                }
+            } catch (IllegalArgumentException e) {
+                throw new JsonParseException(e);
+            }
+        }
+    }
+
+    public static void setDateFormat(DateFormat dateFormat) {
+        dateTypeAdapter.setFormat(dateFormat);
+    }
+
+    public static void setSqlDateFormat(DateFormat dateFormat) {
+        sqlDateTypeAdapter.setFormat(dateFormat);
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/Pair.java

@@ -0,0 +1,57 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class Pair {
+    private String name = "";
+    private String value = "";
+
+    public Pair (String name, String value) {
+        setName(name);
+        setValue(value);
+    }
+
+    private void setName(String name) {
+        if (!isValidString(name)) {
+            return;
+        }
+
+        this.name = name;
+    }
+
+    private void setValue(String value) {
+        if (!isValidString(value)) {
+            return;
+        }
+
+        this.value = value;
+    }
+
+    public String getName() {
+        return this.name;
+    }
+
+    public String getValue() {
+        return this.value;
+    }
+
+    private boolean isValidString(String arg) {
+        if (arg == null) {
+            return false;
+        }
+
+        return true;
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/ProgressRequestBody.java

@@ -0,0 +1,73 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+import okhttp3.MediaType;
+import okhttp3.RequestBody;
+
+import java.io.IOException;
+
+import okio.Buffer;
+import okio.BufferedSink;
+import okio.ForwardingSink;
+import okio.Okio;
+import okio.Sink;
+
+public class ProgressRequestBody extends RequestBody {
+
+    private final RequestBody requestBody;
+
+    private final ApiCallback callback;
+
+    public ProgressRequestBody(RequestBody requestBody, ApiCallback callback) {
+        this.requestBody = requestBody;
+        this.callback = callback;
+    }
+
+    @Override
+    public MediaType contentType() {
+        return requestBody.contentType();
+    }
+
+    @Override
+    public long contentLength() throws IOException {
+        return requestBody.contentLength();
+    }
+
+    @Override
+    public void writeTo(BufferedSink sink) throws IOException {
+        BufferedSink bufferedSink = Okio.buffer(sink(sink));
+        requestBody.writeTo(bufferedSink);
+        bufferedSink.flush();
+    }
+
+    private Sink sink(Sink sink) {
+        return new ForwardingSink(sink) {
+
+            long bytesWritten = 0L;
+            long contentLength = 0L;
+
+            @Override
+            public void write(Buffer source, long byteCount) throws IOException {
+                super.write(source, byteCount);
+                if (contentLength == 0) {
+                    contentLength = contentLength();
+                }
+
+                bytesWritten += byteCount;
+                callback.onUploadProgress(bytesWritten, contentLength, bytesWritten == contentLength);
+            }
+        };
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/ProgressResponseBody.java

@@ -0,0 +1,70 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+import okhttp3.MediaType;
+import okhttp3.ResponseBody;
+
+import java.io.IOException;
+
+import okio.Buffer;
+import okio.BufferedSource;
+import okio.ForwardingSource;
+import okio.Okio;
+import okio.Source;
+
+public class ProgressResponseBody extends ResponseBody {
+
+    private final ResponseBody responseBody;
+    private final ApiCallback callback;
+    private BufferedSource bufferedSource;
+
+    public ProgressResponseBody(ResponseBody responseBody, ApiCallback callback) {
+        this.responseBody = responseBody;
+        this.callback = callback;
+    }
+
+    @Override
+    public MediaType contentType() {
+        return responseBody.contentType();
+    }
+
+    @Override
+    public long contentLength() {
+        return responseBody.contentLength();
+    }
+
+    @Override
+    public BufferedSource source() {
+        if (bufferedSource == null) {
+            bufferedSource = Okio.buffer(source(responseBody.source()));
+        }
+        return bufferedSource;
+    }
+
+    private Source source(Source source) {
+        return new ForwardingSource(source) {
+            long totalBytesRead = 0L;
+
+            @Override
+            public long read(Buffer sink, long byteCount) throws IOException {
+                long bytesRead = super.read(sink, byteCount);
+                // read() returns the number of bytes read, or -1 if this source is exhausted.
+                totalBytesRead += bytesRead != -1 ? bytesRead : 0;
+                callback.onDownloadProgress(totalBytesRead, responseBody.contentLength(), bytesRead == -1);
+                return bytesRead;
+            }
+        };
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/ServerConfiguration.java

@@ -0,0 +1,58 @@
+package org.openapitools.client;
+
+import java.util.Map;
+
+/**
+ * Representing a Server configuration.
+ */
+public class ServerConfiguration {
+    public String URL;
+    public String description;
+    public Map<String, ServerVariable> variables;
+
+    /**
+     * @param URL A URL to the target host.
+     * @param description A description of the host designated by the URL.
+     * @param variables A map between a variable name and its value. The value is used for substitution in the server's URL template.
+     */
+    public ServerConfiguration(String URL, String description, Map<String, ServerVariable> variables) {
+        this.URL = URL;
+        this.description = description;
+        this.variables = variables;
+    }
+
+    /**
+     * Format URL template using given variables.
+     *
+     * @param variables A map between a variable name and its value.
+     * @return Formatted URL.
+     */
+    public String URL(Map<String, String> variables) {
+        String url = this.URL;
+
+        // go through variables and replace placeholders
+        for (Map.Entry<String, ServerVariable> variable: this.variables.entrySet()) {
+            String name = variable.getKey();
+            ServerVariable serverVariable = variable.getValue();
+            String value = serverVariable.defaultValue;
+
+            if (variables != null && variables.containsKey(name)) {
+                value = variables.get(name);
+                if (serverVariable.enumValues.size() > 0 && !serverVariable.enumValues.contains(value)) {
+                    throw new IllegalArgumentException("The variable " + name + " in the server URL has invalid value " + value + ".");
+                }
+            }
+            url = url.replace("{" + name + "}", value);
+        }
+        return url;
+    }
+
+    /**
+     * Format URL template using default server variables.
+     *
+     * @return Formatted URL.
+     */
+    public String URL() {
+        return URL(null);
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/ServerVariable.java

@@ -0,0 +1,23 @@
+package org.openapitools.client;
+
+import java.util.HashSet;
+
+/**
+ * Representing a Server Variable for server URL template substitution.
+ */
+public class ServerVariable {
+    public String description;
+    public String defaultValue;
+    public HashSet<String> enumValues = null;
+
+    /**
+     * @param description A description for the server variable.
+     * @param defaultValue The default value to use for substitution.
+     * @param enumValues An enumeration of string values to be used if the substitution options are from a limited set.
+     */
+    public ServerVariable(String description, String defaultValue, HashSet<String> enumValues) {
+        this.description = description;
+        this.defaultValue = defaultValue;
+        this.enumValues = enumValues;
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/StringUtil.java

@@ -0,0 +1,83 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client;
+
+import java.util.Collection;
+import java.util.Iterator;
+
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class StringUtil {
+  /**
+   * Check if the given array contains the given value (with case-insensitive comparison).
+   *
+   * @param array The array
+   * @param value The value to search
+   * @return true if the array contains the value
+   */
+  public static boolean containsIgnoreCase(String[] array, String value) {
+    for (String str : array) {
+      if (value == null && str == null) {
+        return true;
+      }
+      if (value != null && value.equalsIgnoreCase(str)) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  /**
+   * Join an array of strings with the given separator.
+   * <p>
+   * Note: This might be replaced by utility method from commons-lang or guava someday
+   * if one of those libraries is added as dependency.
+   * </p>
+   *
+   * @param array     The array of strings
+   * @param separator The separator
+   * @return the resulting string
+   */
+  public static String join(String[] array, String separator) {
+    int len = array.length;
+    if (len == 0) {
+      return "";
+    }
+
+    StringBuilder out = new StringBuilder();
+    out.append(array[0]);
+    for (int i = 1; i < len; i++) {
+      out.append(separator).append(array[i]);
+    }
+    return out.toString();
+  }
+
+  /**
+   * Join a list of strings with the given separator.
+   *
+   * @param list      The list of strings
+   * @param separator The separator
+   * @return the resulting string
+   */
+  public static String join(Collection<String> list, String separator) {
+    Iterator<String> iterator = list.iterator();
+    StringBuilder out = new StringBuilder();
+    if (iterator.hasNext()) {
+      out.append(iterator.next());
+    }
+    while (iterator.hasNext()) {
+      out.append(separator).append(iterator.next());
+    }
+    return out.toString();
+  }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/api/StoreApi.java

@@ -0,0 +1,571 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.api;
+
+import org.openapitools.client.ApiCallback;
+import org.openapitools.client.ApiClient;
+import org.openapitools.client.ApiException;
+import org.openapitools.client.ApiResponse;
+import org.openapitools.client.Configuration;
+import org.openapitools.client.Pair;
+import org.openapitools.client.ProgressRequestBody;
+import org.openapitools.client.ProgressResponseBody;
+
+import com.google.gson.reflect.TypeToken;
+
+import java.io.IOException;
+
+
+import org.openapitools.client.model.Order;
+
+import java.lang.reflect.Type;
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import javax.ws.rs.core.GenericType;
+
+public class StoreApi {
+    private ApiClient localVarApiClient;
+    private int localHostIndex;
+    private String localCustomBaseUrl;
+
+    public StoreApi() {
+        this(Configuration.getDefaultApiClient());
+    }
+
+    public StoreApi(ApiClient apiClient) {
+        this.localVarApiClient = apiClient;
+    }
+
+    public ApiClient getApiClient() {
+        return localVarApiClient;
+    }
+
+    public void setApiClient(ApiClient apiClient) {
+        this.localVarApiClient = apiClient;
+    }
+
+    public int getHostIndex() {
+        return localHostIndex;
+    }
+
+    public void setHostIndex(int hostIndex) {
+        this.localHostIndex = hostIndex;
+    }
+
+    public String getCustomBaseUrl() {
+        return localCustomBaseUrl;
+    }
+
+    public void setCustomBaseUrl(String customBaseUrl) {
+        this.localCustomBaseUrl = customBaseUrl;
+    }
+
+    /**
+     * Build call for deleteOrder
+     * @param orderId ID of the order that needs to be deleted (required)
+     * @param _callback Callback for upload/download progress
+     * @return Call to execute
+     * @throws ApiException If fail to serialize the request body object
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 400 </td><td> Invalid ID supplied </td><td>  -  </td></tr>
+        <tr><td> 404 </td><td> Order not found </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call deleteOrderCall(String orderId, final ApiCallback _callback) throws ApiException {
+        String basePath = null;
+        // Operation Servers
+        String[] localBasePaths = new String[] {  };
+
+        // Determine Base Path to Use
+        if (localCustomBaseUrl != null){
+            basePath = localCustomBaseUrl;
+        } else if ( localBasePaths.length > 0 ) {
+            basePath = localBasePaths[localHostIndex];
+        } else {
+            basePath = null;
+        }
+
+        Object localVarPostBody = null;
+
+        // create path and map variables
+        String localVarPath = "/store/order/{orderId}"
+            .replace("{" + "orderId" + "}", localVarApiClient.escapeString(orderId.toString()));
+
+        List<Pair> localVarQueryParams = new ArrayList<Pair>();
+        List<Pair> localVarCollectionQueryParams = new ArrayList<Pair>();
+        Map<String, String> localVarHeaderParams = new HashMap<String, String>();
+        Map<String, String> localVarCookieParams = new HashMap<String, String>();
+        Map<String, Object> localVarFormParams = new HashMap<String, Object>();
+
+        final String[] localVarAccepts = {
+        };
+        final String localVarAccept = localVarApiClient.selectHeaderAccept(localVarAccepts);
+        if (localVarAccept != null) {
+            localVarHeaderParams.put("Accept", localVarAccept);
+        }
+
+        final String[] localVarContentTypes = {
+        };
+        final String localVarContentType = localVarApiClient.selectHeaderContentType(localVarContentTypes);
+        if (localVarContentType != null) {
+            localVarHeaderParams.put("Content-Type", localVarContentType);
+        }
+
+        String[] localVarAuthNames = new String[] {  };
+        return localVarApiClient.buildCall(basePath, localVarPath, "DELETE", localVarQueryParams, localVarCollectionQueryParams, localVarPostBody, localVarHeaderParams, localVarCookieParams, localVarFormParams, localVarAuthNames, _callback);
+    }
+
+    @SuppressWarnings("rawtypes")
+    private okhttp3.Call deleteOrderValidateBeforeCall(String orderId, final ApiCallback _callback) throws ApiException {
+        // verify the required parameter 'orderId' is set
+        if (orderId == null) {
+            throw new ApiException("Missing the required parameter 'orderId' when calling deleteOrder(Async)");
+        }
+
+        return deleteOrderCall(orderId, _callback);
+
+    }
+
+    /**
+     * Delete purchase order by ID
+     * For valid response try integer IDs with value &lt; 1000. Anything above 1000 or nonintegers will generate API errors
+     * @param orderId ID of the order that needs to be deleted (required)
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 400 </td><td> Invalid ID supplied </td><td>  -  </td></tr>
+        <tr><td> 404 </td><td> Order not found </td><td>  -  </td></tr>
+     </table>
+     */
+    public void deleteOrder(String orderId) throws ApiException {
+        deleteOrderWithHttpInfo(orderId);
+    }
+
+    /**
+     * Delete purchase order by ID
+     * For valid response try integer IDs with value &lt; 1000. Anything above 1000 or nonintegers will generate API errors
+     * @param orderId ID of the order that needs to be deleted (required)
+     * @return ApiResponse&lt;Void&gt;
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 400 </td><td> Invalid ID supplied </td><td>  -  </td></tr>
+        <tr><td> 404 </td><td> Order not found </td><td>  -  </td></tr>
+     </table>
+     */
+    public ApiResponse<Void> deleteOrderWithHttpInfo(String orderId) throws ApiException {
+        okhttp3.Call localVarCall = deleteOrderValidateBeforeCall(orderId, null);
+        return localVarApiClient.execute(localVarCall);
+    }
+
+    /**
+     * Delete purchase order by ID (asynchronously)
+     * For valid response try integer IDs with value &lt; 1000. Anything above 1000 or nonintegers will generate API errors
+     * @param orderId ID of the order that needs to be deleted (required)
+     * @param _callback The callback to be executed when the API call finishes
+     * @return The request call
+     * @throws ApiException If fail to process the API call, e.g. serializing the request body object
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 400 </td><td> Invalid ID supplied </td><td>  -  </td></tr>
+        <tr><td> 404 </td><td> Order not found </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call deleteOrderAsync(String orderId, final ApiCallback<Void> _callback) throws ApiException {
+
+        okhttp3.Call localVarCall = deleteOrderValidateBeforeCall(orderId, _callback);
+        localVarApiClient.executeAsync(localVarCall, _callback);
+        return localVarCall;
+    }
+    /**
+     * Build call for getInventory
+     * @param _callback Callback for upload/download progress
+     * @return Call to execute
+     * @throws ApiException If fail to serialize the request body object
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call getInventoryCall(final ApiCallback _callback) throws ApiException {
+        String basePath = null;
+        // Operation Servers
+        String[] localBasePaths = new String[] {  };
+
+        // Determine Base Path to Use
+        if (localCustomBaseUrl != null){
+            basePath = localCustomBaseUrl;
+        } else if ( localBasePaths.length > 0 ) {
+            basePath = localBasePaths[localHostIndex];
+        } else {
+            basePath = null;
+        }
+
+        Object localVarPostBody = null;
+
+        // create path and map variables
+        String localVarPath = "/store/inventory";
+
+        List<Pair> localVarQueryParams = new ArrayList<Pair>();
+        List<Pair> localVarCollectionQueryParams = new ArrayList<Pair>();
+        Map<String, String> localVarHeaderParams = new HashMap<String, String>();
+        Map<String, String> localVarCookieParams = new HashMap<String, String>();
+        Map<String, Object> localVarFormParams = new HashMap<String, Object>();
+
+        final String[] localVarAccepts = {
+            "application/json"
+        };
+        final String localVarAccept = localVarApiClient.selectHeaderAccept(localVarAccepts);
+        if (localVarAccept != null) {
+            localVarHeaderParams.put("Accept", localVarAccept);
+        }
+
+        final String[] localVarContentTypes = {
+        };
+        final String localVarContentType = localVarApiClient.selectHeaderContentType(localVarContentTypes);
+        if (localVarContentType != null) {
+            localVarHeaderParams.put("Content-Type", localVarContentType);
+        }
+
+        String[] localVarAuthNames = new String[] { "api_key" };
+        return localVarApiClient.buildCall(basePath, localVarPath, "GET", localVarQueryParams, localVarCollectionQueryParams, localVarPostBody, localVarHeaderParams, localVarCookieParams, localVarFormParams, localVarAuthNames, _callback);
+    }
+
+    @SuppressWarnings("rawtypes")
+    private okhttp3.Call getInventoryValidateBeforeCall(final ApiCallback _callback) throws ApiException {
+        return getInventoryCall(_callback);
+
+    }
+
+    /**
+     * Returns pet inventories by status
+     * Returns a map of status codes to quantities
+     * @return Map&lt;String, Integer&gt;
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+     </table>
+     */
+    public Map<String, Integer> getInventory() throws ApiException {
+        ApiResponse<Map<String, Integer>> localVarResp = getInventoryWithHttpInfo();
+        return localVarResp.getData();
+    }
+
+    /**
+     * Returns pet inventories by status
+     * Returns a map of status codes to quantities
+     * @return ApiResponse&lt;Map&lt;String, Integer&gt;&gt;
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+     </table>
+     */
+    public ApiResponse<Map<String, Integer>> getInventoryWithHttpInfo() throws ApiException {
+        okhttp3.Call localVarCall = getInventoryValidateBeforeCall(null);
+        Type localVarReturnType = new TypeToken<Map<String, Integer>>(){}.getType();
+        return localVarApiClient.execute(localVarCall, localVarReturnType);
+    }
+
+    /**
+     * Returns pet inventories by status (asynchronously)
+     * Returns a map of status codes to quantities
+     * @param _callback The callback to be executed when the API call finishes
+     * @return The request call
+     * @throws ApiException If fail to process the API call, e.g. serializing the request body object
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call getInventoryAsync(final ApiCallback<Map<String, Integer>> _callback) throws ApiException {
+
+        okhttp3.Call localVarCall = getInventoryValidateBeforeCall(_callback);
+        Type localVarReturnType = new TypeToken<Map<String, Integer>>(){}.getType();
+        localVarApiClient.executeAsync(localVarCall, localVarReturnType, _callback);
+        return localVarCall;
+    }
+    /**
+     * Build call for getOrderById
+     * @param orderId ID of pet that needs to be fetched (required)
+     * @param _callback Callback for upload/download progress
+     * @return Call to execute
+     * @throws ApiException If fail to serialize the request body object
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+        <tr><td> 400 </td><td> Invalid ID supplied </td><td>  -  </td></tr>
+        <tr><td> 404 </td><td> Order not found </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call getOrderByIdCall(Long orderId, final ApiCallback _callback) throws ApiException {
+        String basePath = null;
+        // Operation Servers
+        String[] localBasePaths = new String[] {  };
+
+        // Determine Base Path to Use
+        if (localCustomBaseUrl != null){
+            basePath = localCustomBaseUrl;
+        } else if ( localBasePaths.length > 0 ) {
+            basePath = localBasePaths[localHostIndex];
+        } else {
+            basePath = null;
+        }
+
+        Object localVarPostBody = null;
+
+        // create path and map variables
+        String localVarPath = "/store/order/{orderId}"
+            .replace("{" + "orderId" + "}", localVarApiClient.escapeString(orderId.toString()));
+
+        List<Pair> localVarQueryParams = new ArrayList<Pair>();
+        List<Pair> localVarCollectionQueryParams = new ArrayList<Pair>();
+        Map<String, String> localVarHeaderParams = new HashMap<String, String>();
+        Map<String, String> localVarCookieParams = new HashMap<String, String>();
+        Map<String, Object> localVarFormParams = new HashMap<String, Object>();
+
+        final String[] localVarAccepts = {
+            "application/xml",
+            "application/json"
+        };
+        final String localVarAccept = localVarApiClient.selectHeaderAccept(localVarAccepts);
+        if (localVarAccept != null) {
+            localVarHeaderParams.put("Accept", localVarAccept);
+        }
+
+        final String[] localVarContentTypes = {
+        };
+        final String localVarContentType = localVarApiClient.selectHeaderContentType(localVarContentTypes);
+        if (localVarContentType != null) {
+            localVarHeaderParams.put("Content-Type", localVarContentType);
+        }
+
+        String[] localVarAuthNames = new String[] {  };
+        return localVarApiClient.buildCall(basePath, localVarPath, "GET", localVarQueryParams, localVarCollectionQueryParams, localVarPostBody, localVarHeaderParams, localVarCookieParams, localVarFormParams, localVarAuthNames, _callback);
+    }
+
+    @SuppressWarnings("rawtypes")
+    private okhttp3.Call getOrderByIdValidateBeforeCall(Long orderId, final ApiCallback _callback) throws ApiException {
+        // verify the required parameter 'orderId' is set
+        if (orderId == null) {
+            throw new ApiException("Missing the required parameter 'orderId' when calling getOrderById(Async)");
+        }
+
+        return getOrderByIdCall(orderId, _callback);
+
+    }
+
+    /**
+     * Find purchase order by ID
+     * For valid response try integer IDs with value &lt;&#x3D; 5 or &gt; 10. Other values will generate exceptions
+     * @param orderId ID of pet that needs to be fetched (required)
+     * @return Order
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+        <tr><td> 400 </td><td> Invalid ID supplied </td><td>  -  </td></tr>
+        <tr><td> 404 </td><td> Order not found </td><td>  -  </td></tr>
+     </table>
+     */
+    public Order getOrderById(Long orderId) throws ApiException {
+        ApiResponse<Order> localVarResp = getOrderByIdWithHttpInfo(orderId);
+        return localVarResp.getData();
+    }
+
+    /**
+     * Find purchase order by ID
+     * For valid response try integer IDs with value &lt;&#x3D; 5 or &gt; 10. Other values will generate exceptions
+     * @param orderId ID of pet that needs to be fetched (required)
+     * @return ApiResponse&lt;Order&gt;
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+        <tr><td> 400 </td><td> Invalid ID supplied </td><td>  -  </td></tr>
+        <tr><td> 404 </td><td> Order not found </td><td>  -  </td></tr>
+     </table>
+     */
+    public ApiResponse<Order> getOrderByIdWithHttpInfo(Long orderId) throws ApiException {
+        okhttp3.Call localVarCall = getOrderByIdValidateBeforeCall(orderId, null);
+        Type localVarReturnType = new TypeToken<Order>(){}.getType();
+        return localVarApiClient.execute(localVarCall, localVarReturnType);
+    }
+
+    /**
+     * Find purchase order by ID (asynchronously)
+     * For valid response try integer IDs with value &lt;&#x3D; 5 or &gt; 10. Other values will generate exceptions
+     * @param orderId ID of pet that needs to be fetched (required)
+     * @param _callback The callback to be executed when the API call finishes
+     * @return The request call
+     * @throws ApiException If fail to process the API call, e.g. serializing the request body object
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+        <tr><td> 400 </td><td> Invalid ID supplied </td><td>  -  </td></tr>
+        <tr><td> 404 </td><td> Order not found </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call getOrderByIdAsync(Long orderId, final ApiCallback<Order> _callback) throws ApiException {
+
+        okhttp3.Call localVarCall = getOrderByIdValidateBeforeCall(orderId, _callback);
+        Type localVarReturnType = new TypeToken<Order>(){}.getType();
+        localVarApiClient.executeAsync(localVarCall, localVarReturnType, _callback);
+        return localVarCall;
+    }
+    /**
+     * Build call for placeOrder
+     * @param order order placed for purchasing the pet (required)
+     * @param _callback Callback for upload/download progress
+     * @return Call to execute
+     * @throws ApiException If fail to serialize the request body object
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+        <tr><td> 400 </td><td> Invalid Order </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call placeOrderCall(Order order, final ApiCallback _callback) throws ApiException {
+        String basePath = null;
+        // Operation Servers
+        String[] localBasePaths = new String[] {  };
+
+        // Determine Base Path to Use
+        if (localCustomBaseUrl != null){
+            basePath = localCustomBaseUrl;
+        } else if ( localBasePaths.length > 0 ) {
+            basePath = localBasePaths[localHostIndex];
+        } else {
+            basePath = null;
+        }
+
+        Object localVarPostBody = order;
+
+        // create path and map variables
+        String localVarPath = "/store/order";
+
+        List<Pair> localVarQueryParams = new ArrayList<Pair>();
+        List<Pair> localVarCollectionQueryParams = new ArrayList<Pair>();
+        Map<String, String> localVarHeaderParams = new HashMap<String, String>();
+        Map<String, String> localVarCookieParams = new HashMap<String, String>();
+        Map<String, Object> localVarFormParams = new HashMap<String, Object>();
+
+        final String[] localVarAccepts = {
+            "application/xml",
+            "application/json"
+        };
+        final String localVarAccept = localVarApiClient.selectHeaderAccept(localVarAccepts);
+        if (localVarAccept != null) {
+            localVarHeaderParams.put("Accept", localVarAccept);
+        }
+
+        final String[] localVarContentTypes = {
+            "application/json"
+        };
+        final String localVarContentType = localVarApiClient.selectHeaderContentType(localVarContentTypes);
+        if (localVarContentType != null) {
+            localVarHeaderParams.put("Content-Type", localVarContentType);
+        }
+
+        String[] localVarAuthNames = new String[] {  };
+        return localVarApiClient.buildCall(basePath, localVarPath, "POST", localVarQueryParams, localVarCollectionQueryParams, localVarPostBody, localVarHeaderParams, localVarCookieParams, localVarFormParams, localVarAuthNames, _callback);
+    }
+
+    @SuppressWarnings("rawtypes")
+    private okhttp3.Call placeOrderValidateBeforeCall(Order order, final ApiCallback _callback) throws ApiException {
+        // verify the required parameter 'order' is set
+        if (order == null) {
+            throw new ApiException("Missing the required parameter 'order' when calling placeOrder(Async)");
+        }
+
+        return placeOrderCall(order, _callback);
+
+    }
+
+    /**
+     * Place an order for a pet
+     * 
+     * @param order order placed for purchasing the pet (required)
+     * @return Order
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+        <tr><td> 400 </td><td> Invalid Order </td><td>  -  </td></tr>
+     </table>
+     */
+    public Order placeOrder(Order order) throws ApiException {
+        ApiResponse<Order> localVarResp = placeOrderWithHttpInfo(order);
+        return localVarResp.getData();
+    }
+
+    /**
+     * Place an order for a pet
+     * 
+     * @param order order placed for purchasing the pet (required)
+     * @return ApiResponse&lt;Order&gt;
+     * @throws ApiException If fail to call the API, e.g. server error or cannot deserialize the response body
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+        <tr><td> 400 </td><td> Invalid Order </td><td>  -  </td></tr>
+     </table>
+     */
+    public ApiResponse<Order> placeOrderWithHttpInfo(Order order) throws ApiException {
+        okhttp3.Call localVarCall = placeOrderValidateBeforeCall(order, null);
+        Type localVarReturnType = new TypeToken<Order>(){}.getType();
+        return localVarApiClient.execute(localVarCall, localVarReturnType);
+    }
+
+    /**
+     * Place an order for a pet (asynchronously)
+     * 
+     * @param order order placed for purchasing the pet (required)
+     * @param _callback The callback to be executed when the API call finishes
+     * @return The request call
+     * @throws ApiException If fail to process the API call, e.g. serializing the request body object
+     * @http.response.details
+     <table summary="Response Details" border="1">
+        <tr><td> Status Code </td><td> Description </td><td> Response Headers </td></tr>
+        <tr><td> 200 </td><td> successful operation </td><td>  -  </td></tr>
+        <tr><td> 400 </td><td> Invalid Order </td><td>  -  </td></tr>
+     </table>
+     */
+    public okhttp3.Call placeOrderAsync(Order order, final ApiCallback<Order> _callback) throws ApiException {
+
+        okhttp3.Call localVarCall = placeOrderValidateBeforeCall(order, _callback);
+        Type localVarReturnType = new TypeToken<Order>(){}.getType();
+        localVarApiClient.executeAsync(localVarCall, localVarReturnType, _callback);
+        return localVarCall;
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/auth/ApiKeyAuth.java

@@ -0,0 +1,80 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.auth;
+
+import org.openapitools.client.ApiException;
+import org.openapitools.client.Pair;
+
+import java.net.URI;
+import java.util.Map;
+import java.util.List;
+
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class ApiKeyAuth implements Authentication {
+  private final String location;
+  private final String paramName;
+
+  private String apiKey;
+  private String apiKeyPrefix;
+
+  public ApiKeyAuth(String location, String paramName) {
+    this.location = location;
+    this.paramName = paramName;
+  }
+
+  public String getLocation() {
+    return location;
+  }
+
+  public String getParamName() {
+    return paramName;
+  }
+
+  public String getApiKey() {
+    return apiKey;
+  }
+
+  public void setApiKey(String apiKey) {
+    this.apiKey = apiKey;
+  }
+
+  public String getApiKeyPrefix() {
+    return apiKeyPrefix;
+  }
+
+  public void setApiKeyPrefix(String apiKeyPrefix) {
+    this.apiKeyPrefix = apiKeyPrefix;
+  }
+
+  @Override
+  public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams, Map<String, String> cookieParams,
+                           String payload, String method, URI uri) throws ApiException {
+    if (apiKey == null) {
+      return;
+    }
+    String value;
+    if (apiKeyPrefix != null) {
+      value = apiKeyPrefix + " " + apiKey;
+    } else {
+      value = apiKey;
+    }
+    if ("query".equals(location)) {
+      queryParams.add(new Pair(paramName, value));
+    } else if ("header".equals(location)) {
+      headerParams.put(paramName, value);
+    } else if ("cookie".equals(location)) {
+      cookieParams.put(paramName, value);
+    }
+  }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/auth/Authentication.java

@@ -0,0 +1,36 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.auth;
+
+import org.openapitools.client.Pair;
+import org.openapitools.client.ApiException;
+
+import java.net.URI;
+import java.util.Map;
+import java.util.List;
+
+public interface Authentication {
+    /**
+     * Apply authentication settings to header and query params.
+     *
+     * @param queryParams List of query parameters
+     * @param headerParams Map of header parameters
+     * @param cookieParams Map of cookie parameters
+     * @param payload HTTP request body
+     * @param method HTTP method
+     * @param uri URI
+     * @throws ApiException if failed to update the parameters
+     */
+    void applyToParams(List<Pair> queryParams, Map<String, String> headerParams, Map<String, String> cookieParams, String payload, String method, URI uri) throws ApiException;
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/auth/HttpBasicAuth.java

@@ -0,0 +1,57 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.auth;
+
+import org.openapitools.client.Pair;
+import org.openapitools.client.ApiException;
+
+import okhttp3.Credentials;
+
+import java.net.URI;
+import java.util.Map;
+import java.util.List;
+
+import java.io.UnsupportedEncodingException;
+
+public class HttpBasicAuth implements Authentication {
+    private String username;
+    private String password;
+
+    public String getUsername() {
+        return username;
+    }
+
+    public void setUsername(String username) {
+        this.username = username;
+    }
+
+    public String getPassword() {
+        return password;
+    }
+
+    public void setPassword(String password) {
+        this.password = password;
+    }
+
+    @Override
+    public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams, Map<String, String> cookieParams,
+                              String payload, String method, URI uri) throws ApiException {
+        if (username == null && password == null) {
+            return;
+        }
+        headerParams.put("Authorization", Credentials.basic(
+            username == null ? "" : username,
+            password == null ? "" : password));
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/auth/HttpBearerAuth.java

@@ -0,0 +1,63 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.auth;
+
+import org.openapitools.client.ApiException;
+import org.openapitools.client.Pair;
+
+import java.net.URI;
+import java.util.Map;
+import java.util.List;
+
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class HttpBearerAuth implements Authentication {
+  private final String scheme;
+  private String bearerToken;
+
+  public HttpBearerAuth(String scheme) {
+    this.scheme = scheme;
+  }
+
+  /**
+   * Gets the token, which together with the scheme, will be sent as the value of the Authorization header.
+   *
+   * @return The bearer token
+   */
+  public String getBearerToken() {
+    return bearerToken;
+  }
+
+  /**
+   * Sets the token, which together with the scheme, will be sent as the value of the Authorization header.
+   *
+   * @param bearerToken The bearer token to send in the Authorization header
+   */
+  public void setBearerToken(String bearerToken) {
+    this.bearerToken = bearerToken;
+  }
+
+  @Override
+  public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams, Map<String, String> cookieParams,
+                            String payload, String method, URI uri) throws ApiException {
+    if (bearerToken == null) {
+      return;
+    }
+
+    headerParams.put("Authorization", (scheme != null ? upperCaseBearer(scheme) + " " : "") + bearerToken);
+  }
+
+  private static String upperCaseBearer(String scheme) {
+    return ("bearer".equalsIgnoreCase(scheme)) ? "Bearer" : scheme;
+  }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/auth/OAuth.java

@@ -0,0 +1,42 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.auth;
+
+import org.openapitools.client.Pair;
+import org.openapitools.client.ApiException;
+
+import java.net.URI;
+import java.util.Map;
+import java.util.List;
+
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class OAuth implements Authentication {
+  private String accessToken;
+
+  public String getAccessToken() {
+    return accessToken;
+  }
+
+  public void setAccessToken(String accessToken) {
+    this.accessToken = accessToken;
+  }
+
+  @Override
+  public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams, Map<String, String> cookieParams,
+                            String payload, String method, URI uri) throws ApiException {
+    if (accessToken != null) {
+      headerParams.put("Authorization", "Bearer " + accessToken);
+    }
+  }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/auth/OAuthFlow.java

@@ -0,0 +1,25 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.auth;
+
+/**
+ * OAuth flows that are supported by this client
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public enum OAuthFlow {
+    ACCESS_CODE, //called authorizationCode  in OpenAPI 3.0
+    IMPLICIT,
+    PASSWORD,
+    APPLICATION //called clientCredentials in OpenAPI 3.0
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/auth/OAuthOkHttpClient.java

@@ -0,0 +1,69 @@
+package org.openapitools.client.auth;
+
+import okhttp3.OkHttpClient;
+import okhttp3.MediaType;
+import okhttp3.Request;
+import okhttp3.RequestBody;
+import okhttp3.Response;
+
+import org.apache.oltu.oauth2.client.HttpClient;
+import org.apache.oltu.oauth2.client.request.OAuthClientRequest;
+import org.apache.oltu.oauth2.client.response.OAuthClientResponse;
+import org.apache.oltu.oauth2.client.response.OAuthClientResponseFactory;
+import org.apache.oltu.oauth2.common.exception.OAuthProblemException;
+import org.apache.oltu.oauth2.common.exception.OAuthSystemException;
+
+import java.io.IOException;
+import java.util.Map;
+import java.util.Map.Entry;
+
+public class OAuthOkHttpClient implements HttpClient {
+    private OkHttpClient client;
+
+    public OAuthOkHttpClient() {
+        this.client = new OkHttpClient();
+    }
+
+    public OAuthOkHttpClient(OkHttpClient client) {
+        this.client = client;
+    }
+
+    @Override
+    public <T extends OAuthClientResponse> T execute(OAuthClientRequest request, Map<String, String> headers,
+                                                     String requestMethod, Class<T> responseClass)
+            throws OAuthSystemException, OAuthProblemException {
+
+        MediaType mediaType = MediaType.parse("application/json");
+        Request.Builder requestBuilder = new Request.Builder().url(request.getLocationUri());
+
+        if(headers != null) {
+            for (Entry<String, String> entry : headers.entrySet()) {
+                if (entry.getKey().equalsIgnoreCase("Content-Type")) {
+                    mediaType = MediaType.parse(entry.getValue());
+                } else {
+                    requestBuilder.addHeader(entry.getKey(), entry.getValue());
+                }
+            }
+        }
+
+        RequestBody body = request.getBody() != null ? RequestBody.create(request.getBody(), mediaType) : null;
+        requestBuilder.method(requestMethod, body);
+
+        try {
+            Response response = client.newCall(requestBuilder.build()).execute();
+            return OAuthClientResponseFactory.createCustomResponse(
+                    response.body().string(),
+                    response.body().contentType().toString(),
+                    response.code(),
+                    response.headers().toMultimap(),
+                    responseClass);
+        } catch (IOException e) {
+            throw new OAuthSystemException(e);
+        }
+    }
+
+    @Override
+    public void shutdown() {
+        // Nothing to do here
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/auth/RetryingOAuth.java

@@ -0,0 +1,210 @@
+package org.openapitools.client.auth;
+
+import org.openapitools.client.ApiException;
+import org.openapitools.client.Pair;
+
+import okhttp3.Interceptor;
+import okhttp3.OkHttpClient;
+import okhttp3.Request;
+import okhttp3.Response;
+
+import org.apache.oltu.oauth2.client.OAuthClient;
+import org.apache.oltu.oauth2.client.request.OAuthBearerClientRequest;
+import org.apache.oltu.oauth2.client.request.OAuthClientRequest;
+import org.apache.oltu.oauth2.client.request.OAuthClientRequest.TokenRequestBuilder;
+import org.apache.oltu.oauth2.client.response.OAuthJSONAccessTokenResponse;
+import org.apache.oltu.oauth2.common.exception.OAuthProblemException;
+import org.apache.oltu.oauth2.common.exception.OAuthSystemException;
+import org.apache.oltu.oauth2.common.message.types.GrantType;
+
+import java.io.IOException;
+import java.net.HttpURLConnection;
+import java.net.URI;
+import java.util.Map;
+import java.util.List;
+
+public class RetryingOAuth extends OAuth implements Interceptor {
+    private OAuthClient oAuthClient;
+
+    private TokenRequestBuilder tokenRequestBuilder;
+
+    /**
+     * @param client An OkHttp client
+     * @param tokenRequestBuilder A token request builder
+     */
+    public RetryingOAuth(OkHttpClient client, TokenRequestBuilder tokenRequestBuilder) {
+        this.oAuthClient = new OAuthClient(new OAuthOkHttpClient(client));
+        this.tokenRequestBuilder = tokenRequestBuilder;
+    }
+
+    /**
+     * @param tokenRequestBuilder A token request builder
+     */
+    public RetryingOAuth(TokenRequestBuilder tokenRequestBuilder) {
+        this(new OkHttpClient(), tokenRequestBuilder);
+    }
+
+    /**
+     * @param tokenUrl The token URL to be used for this OAuth2 flow.
+     *   Applicable to the following OAuth2 flows: "password", "clientCredentials" and "authorizationCode".
+     *   The value must be an absolute URL.
+     * @param clientId The OAuth2 client ID for the "clientCredentials" flow.
+     * @param flow OAuth flow.
+     * @param clientSecret The OAuth2 client secret for the "clientCredentials" flow.
+     * @param parameters A map of string.
+     */
+    public RetryingOAuth(
+            String tokenUrl,
+            String clientId,
+            OAuthFlow flow,
+            String clientSecret,
+            Map<String, String> parameters
+    ) {
+        this(OAuthClientRequest.tokenLocation(tokenUrl)
+                .setClientId(clientId)
+                .setClientSecret(clientSecret));
+        setFlow(flow);
+        if (parameters != null) {
+            for (Map.Entry<String, String> entry : parameters.entrySet()) {
+                tokenRequestBuilder.setParameter(entry.getKey(), entry.getValue());
+            }
+        }
+    }
+
+    /**
+     * Set the OAuth flow
+     *
+     * @param flow The OAuth flow.
+     */
+    public void setFlow(OAuthFlow flow) {
+        switch(flow) {
+            case ACCESS_CODE:
+                tokenRequestBuilder.setGrantType(GrantType.AUTHORIZATION_CODE);
+                break;
+            case IMPLICIT:
+                tokenRequestBuilder.setGrantType(GrantType.IMPLICIT);
+                break;
+            case PASSWORD:
+                tokenRequestBuilder.setGrantType(GrantType.PASSWORD);
+                break;
+            case APPLICATION:
+                tokenRequestBuilder.setGrantType(GrantType.CLIENT_CREDENTIALS);
+                break;
+            default:
+                break;
+        }
+    }
+
+    @Override
+    public Response intercept(Chain chain) throws IOException {
+        return retryingIntercept(chain, true);
+    }
+
+    private Response retryingIntercept(Chain chain, boolean updateTokenAndRetryOnAuthorizationFailure) throws IOException {
+        Request request = chain.request();
+
+        // If the request already has an authorization (e.g. Basic auth), proceed with the request as is
+        if (request.header("Authorization") != null) {
+            return chain.proceed(request);
+        }
+
+        // Get the token if it has not yet been acquired
+        if (getAccessToken() == null) {
+            updateAccessToken(null);
+        }
+
+        OAuthClientRequest oAuthRequest;
+        if (getAccessToken() != null) {
+            // Build the request
+            Request.Builder requestBuilder = request.newBuilder();
+
+            String requestAccessToken = getAccessToken();
+            try {
+                oAuthRequest =
+                        new OAuthBearerClientRequest(request.url().toString()).
+                                setAccessToken(requestAccessToken).
+                                buildHeaderMessage();
+            } catch (OAuthSystemException e) {
+                throw new IOException(e);
+            }
+
+            Map<String, String> headers = oAuthRequest.getHeaders();
+            for (Map.Entry<String, String> entry : headers.entrySet()) {
+                requestBuilder.addHeader(entry.getKey(), entry.getValue());
+            }
+            requestBuilder.url(oAuthRequest.getLocationUri());
+
+            // Execute the request
+            Response response = chain.proceed(requestBuilder.build());
+
+            // 401/403 response codes most likely indicate an expired access token, unless it happens two times in a row
+            if (
+                    response != null &&
+                            (   response.code() == HttpURLConnection.HTTP_UNAUTHORIZED ||
+                                    response.code() == HttpURLConnection.HTTP_FORBIDDEN     ) &&
+                            updateTokenAndRetryOnAuthorizationFailure
+            ) {
+                try {
+                    if (updateAccessToken(requestAccessToken)) {
+                        response.body().close();
+                        return retryingIntercept(chain, false);
+                    }
+                } catch (Exception e) {
+                    response.body().close();
+                    throw e;
+                }
+            }
+            return response;
+        }
+        else {
+            return chain.proceed(chain.request());
+        }
+    }
+
+    /**
+     * Returns true if the access token has been updated
+     *
+     * @param requestAccessToken the request access token
+     * @return True if the update is successful
+     * @throws java.io.IOException If fail to update the access token
+     */
+    public synchronized boolean updateAccessToken(String requestAccessToken) throws IOException {
+        if (getAccessToken() == null || getAccessToken().equals(requestAccessToken)) {
+            try {
+                OAuthJSONAccessTokenResponse accessTokenResponse =
+                        oAuthClient.accessToken(tokenRequestBuilder.buildBodyMessage());
+                if (accessTokenResponse != null && accessTokenResponse.getAccessToken() != null) {
+                    setAccessToken(accessTokenResponse.getAccessToken());
+                }
+            } catch (OAuthSystemException | OAuthProblemException e) {
+                throw new IOException(e);
+            }
+        }
+        return getAccessToken() == null || !getAccessToken().equals(requestAccessToken);
+    }
+
+    /**
+     * Gets the token request builder
+     *
+     * @return A token request builder
+     */
+    public TokenRequestBuilder getTokenRequestBuilder() {
+        return tokenRequestBuilder;
+    }
+
+    /**
+     * Sets the token request builder
+     *
+     * @param tokenRequestBuilder Token request builder
+     */
+    public void setTokenRequestBuilder(TokenRequestBuilder tokenRequestBuilder) {
+        this.tokenRequestBuilder = tokenRequestBuilder;
+    }
+
+    // Applying authorization to parameters is performed in the retryingIntercept method
+    @Override
+    public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams, Map<String, String> cookieParams,
+                             String payload, String method, URI uri) throws ApiException {
+        // No implementation necessary
+    }
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/AbstractOpenApiSchema.java

@@ -0,0 +1,149 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import org.openapitools.client.ApiException;
+import java.util.Objects;
+import java.lang.reflect.Type;
+import java.util.Map;
+import javax.ws.rs.core.GenericType;
+
+//import com.fasterxml.jackson.annotation.JsonValue;
+
+/**
+ * Abstract class for oneOf,anyOf schemas defined in OpenAPI spec
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public abstract class AbstractOpenApiSchema {
+
+    // store the actual instance of the schema/object
+    private Object instance;
+
+    // is nullable
+    private Boolean isNullable;
+
+    // schema type (e.g. oneOf, anyOf)
+    private final String schemaType;
+
+    public AbstractOpenApiSchema(String schemaType, Boolean isNullable) {
+        this.schemaType = schemaType;
+        this.isNullable = isNullable;
+    }
+
+    /**
+     * Get the list of oneOf/anyOf composed schemas allowed to be stored in this object
+     *
+     * @return an instance of the actual schema/object
+     */
+    public abstract Map<String, GenericType> getSchemas();
+
+    /**
+     * Get the actual instance
+     *
+     * @return an instance of the actual schema/object
+     */
+    //@JsonValue
+    public Object getActualInstance() {return instance;}
+
+    /**
+     * Set the actual instance
+     *
+     * @param instance the actual instance of the schema/object
+     */
+    public void setActualInstance(Object instance) {this.instance = instance;}
+
+    /**
+     * Get the instant recursively when the schemas defined in oneOf/anyof happen to be oneOf/anyOf schema as well
+     *
+     * @return an instance of the actual schema/object
+     */
+    public Object getActualInstanceRecursively() {
+        return getActualInstanceRecursively(this);
+    }
+
+    private Object getActualInstanceRecursively(AbstractOpenApiSchema object) {
+        if (object.getActualInstance() == null) {
+            return null;
+        } else if (object.getActualInstance() instanceof AbstractOpenApiSchema) {
+            return getActualInstanceRecursively((AbstractOpenApiSchema)object.getActualInstance());
+        } else {
+            return object.getActualInstance();
+        }
+    }
+
+    /**
+     * Get the schema type (e.g. anyOf, oneOf)
+     *
+     * @return the schema type
+     */
+    public String getSchemaType() {
+        return schemaType;
+    }
+
+    @Override
+    public String toString() {
+        StringBuilder sb = new StringBuilder();
+        sb.append("class ").append(getClass()).append(" {\n");
+        sb.append("    instance: ").append(toIndentedString(instance)).append("\n");
+        sb.append("    isNullable: ").append(toIndentedString(isNullable)).append("\n");
+        sb.append("    schemaType: ").append(toIndentedString(schemaType)).append("\n");
+        sb.append("}");
+        return sb.toString();
+    }
+
+    /**
+     * Convert the given object to string with each line indented by 4 spaces
+     * (except the first line).
+     */
+    private String toIndentedString(Object o) {
+        if (o == null) {
+            return "null";
+        }
+        return o.toString().replace("\n", "\n    ");
+    }
+
+    public boolean equals(Object o) {
+        if (this == o) {
+            return true;
+        }
+        if (o == null || getClass() != o.getClass()) {
+            return false;
+        }
+        AbstractOpenApiSchema a = (AbstractOpenApiSchema) o;
+        return Objects.equals(this.instance, a.instance) &&
+            Objects.equals(this.isNullable, a.isNullable) &&
+            Objects.equals(this.schemaType, a.schemaType);
+    }
+
+    @Override
+    public int hashCode() {
+        return Objects.hash(instance, isNullable, schemaType);
+    }
+
+    /**
+     * Is nullable
+     *
+     * @return true if it's nullable
+     */
+    public Boolean isNullable() {
+        if (Boolean.TRUE.equals(isNullable)) {
+            return Boolean.TRUE;
+        } else {
+            return Boolean.FALSE;
+        }
+    }
+
+
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/Category.java

@@ -0,0 +1,311 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import java.util.Arrays;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * A category for a pet
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class Category {
+  public static final String SERIALIZED_NAME_ID = "id";
+  @SerializedName(SERIALIZED_NAME_ID)
+  private Long id;
+
+  public static final String SERIALIZED_NAME_NAME = "name";
+  @SerializedName(SERIALIZED_NAME_NAME)
+  private String name;
+
+  public Category() {
+  }
+
+  public Category id(Long id) {
+    
+    this.id = id;
+    return this;
+  }
+
+   /**
+   * Get id
+   * @return id
+  **/
+  @javax.annotation.Nullable
+  public Long getId() {
+    return id;
+  }
+
+
+  public void setId(Long id) {
+    this.id = id;
+  }
+
+
+  public Category name(String name) {
+    
+    this.name = name;
+    return this;
+  }
+
+   /**
+   * Get name
+   * @return name
+  **/
+  @javax.annotation.Nullable
+  public String getName() {
+    return name;
+  }
+
+
+  public void setName(String name) {
+    this.name = name;
+  }
+
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the Category instance itself
+   */
+  public Category putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    Category category = (Category) o;
+    return Objects.equals(this.id, category.id) &&
+        Objects.equals(this.name, category.name)&&
+        Objects.equals(this.additionalProperties, category.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(id, name, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class Category {\n");
+    sb.append("    id: ").append(toIndentedString(id)).append("\n");
+    sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>();
+    openapiFields.add("id");
+    openapiFields.add("name");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+  }
+
+ /**
+  * Validates the JSON Object and throws an exception if issues found
+  *
+  * @param jsonObj JSON Object
+  * @throws IOException if the JSON Object is invalid with respect to Category
+  */
+  public static void validateJsonObject(JsonObject jsonObj) throws IOException {
+      if (jsonObj == null) {
+        if (!Category.openapiRequiredFields.isEmpty()) { // has required fields but JSON object is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in Category is not found in the empty JSON string", Category.openapiRequiredFields.toString()));
+        }
+      }
+      if ((jsonObj.get("name") != null && !jsonObj.get("name").isJsonNull()) && !jsonObj.get("name").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `name` to be a primitive type in the JSON string but got `%s`", jsonObj.get("name").toString()));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!Category.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'Category' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<Category> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(Category.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<Category>() {
+           @Override
+           public void write(JsonWriter out, Category value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public Category read(JsonReader in) throws IOException {
+             JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
+             validateJsonObject(jsonObj);
+             // store additional fields in the deserialized instance
+             Category instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+ /**
+  * Create an instance of Category given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of Category
+  * @throws IOException if the JSON string is invalid with respect to Category
+  */
+  public static Category fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, Category.class);
+  }
+
+ /**
+  * Convert an instance of Category to an JSON string
+  *
+  * @return JSON string
+  */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/ModelApiResponse.java

@@ -0,0 +1,342 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import java.util.Arrays;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * Describes the result of uploading an image resource
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class ModelApiResponse {
+  public static final String SERIALIZED_NAME_CODE = "code";
+  @SerializedName(SERIALIZED_NAME_CODE)
+  private Integer code;
+
+  public static final String SERIALIZED_NAME_TYPE = "type";
+  @SerializedName(SERIALIZED_NAME_TYPE)
+  private String type;
+
+  public static final String SERIALIZED_NAME_MESSAGE = "message";
+  @SerializedName(SERIALIZED_NAME_MESSAGE)
+  private String message;
+
+  public ModelApiResponse() {
+  }
+
+  public ModelApiResponse code(Integer code) {
+    
+    this.code = code;
+    return this;
+  }
+
+   /**
+   * Get code
+   * @return code
+  **/
+  @javax.annotation.Nullable
+  public Integer getCode() {
+    return code;
+  }
+
+
+  public void setCode(Integer code) {
+    this.code = code;
+  }
+
+
+  public ModelApiResponse type(String type) {
+    
+    this.type = type;
+    return this;
+  }
+
+   /**
+   * Get type
+   * @return type
+  **/
+  @javax.annotation.Nullable
+  public String getType() {
+    return type;
+  }
+
+
+  public void setType(String type) {
+    this.type = type;
+  }
+
+
+  public ModelApiResponse message(String message) {
+    
+    this.message = message;
+    return this;
+  }
+
+   /**
+   * Get message
+   * @return message
+  **/
+  @javax.annotation.Nullable
+  public String getMessage() {
+    return message;
+  }
+
+
+  public void setMessage(String message) {
+    this.message = message;
+  }
+
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the ModelApiResponse instance itself
+   */
+  public ModelApiResponse putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    ModelApiResponse _apiResponse = (ModelApiResponse) o;
+    return Objects.equals(this.code, _apiResponse.code) &&
+        Objects.equals(this.type, _apiResponse.type) &&
+        Objects.equals(this.message, _apiResponse.message)&&
+        Objects.equals(this.additionalProperties, _apiResponse.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(code, type, message, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class ModelApiResponse {\n");
+    sb.append("    code: ").append(toIndentedString(code)).append("\n");
+    sb.append("    type: ").append(toIndentedString(type)).append("\n");
+    sb.append("    message: ").append(toIndentedString(message)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>();
+    openapiFields.add("code");
+    openapiFields.add("type");
+    openapiFields.add("message");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+  }
+
+ /**
+  * Validates the JSON Object and throws an exception if issues found
+  *
+  * @param jsonObj JSON Object
+  * @throws IOException if the JSON Object is invalid with respect to ModelApiResponse
+  */
+  public static void validateJsonObject(JsonObject jsonObj) throws IOException {
+      if (jsonObj == null) {
+        if (!ModelApiResponse.openapiRequiredFields.isEmpty()) { // has required fields but JSON object is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in ModelApiResponse is not found in the empty JSON string", ModelApiResponse.openapiRequiredFields.toString()));
+        }
+      }
+      if ((jsonObj.get("type") != null && !jsonObj.get("type").isJsonNull()) && !jsonObj.get("type").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `type` to be a primitive type in the JSON string but got `%s`", jsonObj.get("type").toString()));
+      }
+      if ((jsonObj.get("message") != null && !jsonObj.get("message").isJsonNull()) && !jsonObj.get("message").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `message` to be a primitive type in the JSON string but got `%s`", jsonObj.get("message").toString()));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!ModelApiResponse.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'ModelApiResponse' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<ModelApiResponse> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(ModelApiResponse.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<ModelApiResponse>() {
+           @Override
+           public void write(JsonWriter out, ModelApiResponse value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public ModelApiResponse read(JsonReader in) throws IOException {
+             JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
+             validateJsonObject(jsonObj);
+             // store additional fields in the deserialized instance
+             ModelApiResponse instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+ /**
+  * Create an instance of ModelApiResponse given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of ModelApiResponse
+  * @throws IOException if the JSON string is invalid with respect to ModelApiResponse
+  */
+  public static ModelApiResponse fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, ModelApiResponse.class);
+  }
+
+ /**
+  * Convert an instance of ModelApiResponse to an JSON string
+  *
+  * @return JSON string
+  */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/Order.java

@@ -0,0 +1,473 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import java.util.Arrays;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.time.OffsetDateTime;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * An order for a pets from the pet store
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class Order {
+  public static final String SERIALIZED_NAME_ID = "id";
+  @SerializedName(SERIALIZED_NAME_ID)
+  private Long id;
+
+  public static final String SERIALIZED_NAME_PET_ID = "petId";
+  @SerializedName(SERIALIZED_NAME_PET_ID)
+  private Long petId;
+
+  public static final String SERIALIZED_NAME_QUANTITY = "quantity";
+  @SerializedName(SERIALIZED_NAME_QUANTITY)
+  private Integer quantity;
+
+  public static final String SERIALIZED_NAME_SHIP_DATE = "shipDate";
+  @SerializedName(SERIALIZED_NAME_SHIP_DATE)
+  private OffsetDateTime shipDate;
+
+  /**
+   * Order Status
+   */
+  @JsonAdapter(StatusEnum.Adapter.class)
+  public enum StatusEnum {
+    PLACED("placed"),
+    
+    APPROVED("approved"),
+    
+    DELIVERED("delivered");
+
+    private String value;
+
+    StatusEnum(String value) {
+      this.value = value;
+    }
+
+    public String getValue() {
+      return value;
+    }
+
+    @Override
+    public String toString() {
+      return String.valueOf(value);
+    }
+
+    public static StatusEnum fromValue(String value) {
+      for (StatusEnum b : StatusEnum.values()) {
+        if (b.value.equals(value)) {
+          return b;
+        }
+      }
+      throw new IllegalArgumentException("Unexpected value '" + value + "'");
+    }
+
+    public static class Adapter extends TypeAdapter<StatusEnum> {
+      @Override
+      public void write(final JsonWriter jsonWriter, final StatusEnum enumeration) throws IOException {
+        jsonWriter.value(enumeration.getValue());
+      }
+
+      @Override
+      public StatusEnum read(final JsonReader jsonReader) throws IOException {
+        String value =  jsonReader.nextString();
+        return StatusEnum.fromValue(value);
+      }
+    }
+  }
+
+  public static final String SERIALIZED_NAME_STATUS = "status";
+  @SerializedName(SERIALIZED_NAME_STATUS)
+  private StatusEnum status;
+
+  public static final String SERIALIZED_NAME_COMPLETE = "complete";
+  @SerializedName(SERIALIZED_NAME_COMPLETE)
+  private Boolean complete = false;
+
+  public Order() {
+  }
+
+  public Order id(Long id) {
+    
+    this.id = id;
+    return this;
+  }
+
+   /**
+   * Get id
+   * @return id
+  **/
+  @javax.annotation.Nullable
+  public Long getId() {
+    return id;
+  }
+
+
+  public void setId(Long id) {
+    this.id = id;
+  }
+
+
+  public Order petId(Long petId) {
+    
+    this.petId = petId;
+    return this;
+  }
+
+   /**
+   * Get petId
+   * @return petId
+  **/
+  @javax.annotation.Nullable
+  public Long getPetId() {
+    return petId;
+  }
+
+
+  public void setPetId(Long petId) {
+    this.petId = petId;
+  }
+
+
+  public Order quantity(Integer quantity) {
+    
+    this.quantity = quantity;
+    return this;
+  }
+
+   /**
+   * Get quantity
+   * @return quantity
+  **/
+  @javax.annotation.Nullable
+  public Integer getQuantity() {
+    return quantity;
+  }
+
+
+  public void setQuantity(Integer quantity) {
+    this.quantity = quantity;
+  }
+
+
+  public Order shipDate(OffsetDateTime shipDate) {
+    
+    this.shipDate = shipDate;
+    return this;
+  }
+
+   /**
+   * Get shipDate
+   * @return shipDate
+  **/
+  @javax.annotation.Nullable
+  public OffsetDateTime getShipDate() {
+    return shipDate;
+  }
+
+
+  public void setShipDate(OffsetDateTime shipDate) {
+    this.shipDate = shipDate;
+  }
+
+
+  public Order status(StatusEnum status) {
+    
+    this.status = status;
+    return this;
+  }
+
+   /**
+   * Order Status
+   * @return status
+  **/
+  @javax.annotation.Nullable
+  public StatusEnum getStatus() {
+    return status;
+  }
+
+
+  public void setStatus(StatusEnum status) {
+    this.status = status;
+  }
+
+
+  public Order complete(Boolean complete) {
+    
+    this.complete = complete;
+    return this;
+  }
+
+   /**
+   * Get complete
+   * @return complete
+  **/
+  @javax.annotation.Nullable
+  public Boolean getComplete() {
+    return complete;
+  }
+
+
+  public void setComplete(Boolean complete) {
+    this.complete = complete;
+  }
+
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the Order instance itself
+   */
+  public Order putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    Order order = (Order) o;
+    return Objects.equals(this.id, order.id) &&
+        Objects.equals(this.petId, order.petId) &&
+        Objects.equals(this.quantity, order.quantity) &&
+        Objects.equals(this.shipDate, order.shipDate) &&
+        Objects.equals(this.status, order.status) &&
+        Objects.equals(this.complete, order.complete)&&
+        Objects.equals(this.additionalProperties, order.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(id, petId, quantity, shipDate, status, complete, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class Order {\n");
+    sb.append("    id: ").append(toIndentedString(id)).append("\n");
+    sb.append("    petId: ").append(toIndentedString(petId)).append("\n");
+    sb.append("    quantity: ").append(toIndentedString(quantity)).append("\n");
+    sb.append("    shipDate: ").append(toIndentedString(shipDate)).append("\n");
+    sb.append("    status: ").append(toIndentedString(status)).append("\n");
+    sb.append("    complete: ").append(toIndentedString(complete)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>();
+    openapiFields.add("id");
+    openapiFields.add("petId");
+    openapiFields.add("quantity");
+    openapiFields.add("shipDate");
+    openapiFields.add("status");
+    openapiFields.add("complete");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+  }
+
+ /**
+  * Validates the JSON Object and throws an exception if issues found
+  *
+  * @param jsonObj JSON Object
+  * @throws IOException if the JSON Object is invalid with respect to Order
+  */
+  public static void validateJsonObject(JsonObject jsonObj) throws IOException {
+      if (jsonObj == null) {
+        if (!Order.openapiRequiredFields.isEmpty()) { // has required fields but JSON object is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in Order is not found in the empty JSON string", Order.openapiRequiredFields.toString()));
+        }
+      }
+      if ((jsonObj.get("status") != null && !jsonObj.get("status").isJsonNull()) && !jsonObj.get("status").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `status` to be a primitive type in the JSON string but got `%s`", jsonObj.get("status").toString()));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!Order.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'Order' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<Order> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(Order.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<Order>() {
+           @Override
+           public void write(JsonWriter out, Order value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public Order read(JsonReader in) throws IOException {
+             JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
+             validateJsonObject(jsonObj);
+             // store additional fields in the deserialized instance
+             Order instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+ /**
+  * Create an instance of Order given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of Order
+  * @throws IOException if the JSON string is invalid with respect to Order
+  */
+  public static Order fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, Order.class);
+  }
+
+ /**
+  * Convert an instance of Order to an JSON string
+  *
+  * @return JSON string
+  */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/Pet.java

@@ -0,0 +1,533 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import java.util.Arrays;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import org.openapitools.client.model.Category;
+import org.openapitools.client.model.Tag;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * A pet for sale in the pet store
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class Pet {
+  public static final String SERIALIZED_NAME_ID = "id";
+  @SerializedName(SERIALIZED_NAME_ID)
+  private Long id;
+
+  public static final String SERIALIZED_NAME_CATEGORY = "category";
+  @SerializedName(SERIALIZED_NAME_CATEGORY)
+  private Category category;
+
+  public static final String SERIALIZED_NAME_NAME = "name";
+  @SerializedName(SERIALIZED_NAME_NAME)
+  private String name;
+
+  public static final String SERIALIZED_NAME_PHOTO_URLS = "photoUrls";
+  @SerializedName(SERIALIZED_NAME_PHOTO_URLS)
+  private List<String> photoUrls = new ArrayList<>();
+
+  public static final String SERIALIZED_NAME_TAGS = "tags";
+  @SerializedName(SERIALIZED_NAME_TAGS)
+  private List<Tag> tags;
+
+  /**
+   * pet status in the store
+   */
+  @JsonAdapter(StatusEnum.Adapter.class)
+  public enum StatusEnum {
+    AVAILABLE("available"),
+    
+    PENDING("pending"),
+    
+    SOLD("sold");
+
+    private String value;
+
+    StatusEnum(String value) {
+      this.value = value;
+    }
+
+    public String getValue() {
+      return value;
+    }
+
+    @Override
+    public String toString() {
+      return String.valueOf(value);
+    }
+
+    public static StatusEnum fromValue(String value) {
+      for (StatusEnum b : StatusEnum.values()) {
+        if (b.value.equals(value)) {
+          return b;
+        }
+      }
+      throw new IllegalArgumentException("Unexpected value '" + value + "'");
+    }
+
+    public static class Adapter extends TypeAdapter<StatusEnum> {
+      @Override
+      public void write(final JsonWriter jsonWriter, final StatusEnum enumeration) throws IOException {
+        jsonWriter.value(enumeration.getValue());
+      }
+
+      @Override
+      public StatusEnum read(final JsonReader jsonReader) throws IOException {
+        String value =  jsonReader.nextString();
+        return StatusEnum.fromValue(value);
+      }
+    }
+  }
+
+  public static final String SERIALIZED_NAME_STATUS = "status";
+  @Deprecated
+  @SerializedName(SERIALIZED_NAME_STATUS)
+  private StatusEnum status;
+
+  public Pet() {
+  }
+
+  public Pet id(Long id) {
+    
+    this.id = id;
+    return this;
+  }
+
+   /**
+   * Get id
+   * @return id
+  **/
+  @javax.annotation.Nullable
+  public Long getId() {
+    return id;
+  }
+
+
+  public void setId(Long id) {
+    this.id = id;
+  }
+
+
+  public Pet category(Category category) {
+    
+    this.category = category;
+    return this;
+  }
+
+   /**
+   * Get category
+   * @return category
+  **/
+  @javax.annotation.Nullable
+  public Category getCategory() {
+    return category;
+  }
+
+
+  public void setCategory(Category category) {
+    this.category = category;
+  }
+
+
+  public Pet name(String name) {
+    
+    this.name = name;
+    return this;
+  }
+
+   /**
+   * Get name
+   * @return name
+  **/
+  @javax.annotation.Nonnull
+  public String getName() {
+    return name;
+  }
+
+
+  public void setName(String name) {
+    this.name = name;
+  }
+
+
+  public Pet photoUrls(List<String> photoUrls) {
+    
+    this.photoUrls = photoUrls;
+    return this;
+  }
+
+  public Pet addPhotoUrlsItem(String photoUrlsItem) {
+    if (this.photoUrls == null) {
+      this.photoUrls = new ArrayList<>();
+    }
+    this.photoUrls.add(photoUrlsItem);
+    return this;
+  }
+
+   /**
+   * Get photoUrls
+   * @return photoUrls
+  **/
+  @javax.annotation.Nonnull
+  public List<String> getPhotoUrls() {
+    return photoUrls;
+  }
+
+
+  public void setPhotoUrls(List<String> photoUrls) {
+    this.photoUrls = photoUrls;
+  }
+
+
+  public Pet tags(List<Tag> tags) {
+    
+    this.tags = tags;
+    return this;
+  }
+
+  public Pet addTagsItem(Tag tagsItem) {
+    if (this.tags == null) {
+      this.tags = new ArrayList<>();
+    }
+    this.tags.add(tagsItem);
+    return this;
+  }
+
+   /**
+   * Get tags
+   * @return tags
+  **/
+  @javax.annotation.Nullable
+  public List<Tag> getTags() {
+    return tags;
+  }
+
+
+  public void setTags(List<Tag> tags) {
+    this.tags = tags;
+  }
+
+
+  @Deprecated
+  public Pet status(StatusEnum status) {
+    
+    this.status = status;
+    return this;
+  }
+
+   /**
+   * pet status in the store
+   * @return status
+   * @deprecated
+  **/
+  @Deprecated
+  @javax.annotation.Nullable
+  public StatusEnum getStatus() {
+    return status;
+  }
+
+
+  @Deprecated
+  public void setStatus(StatusEnum status) {
+    this.status = status;
+  }
+
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the Pet instance itself
+   */
+  public Pet putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    Pet pet = (Pet) o;
+    return Objects.equals(this.id, pet.id) &&
+        Objects.equals(this.category, pet.category) &&
+        Objects.equals(this.name, pet.name) &&
+        Objects.equals(this.photoUrls, pet.photoUrls) &&
+        Objects.equals(this.tags, pet.tags) &&
+        Objects.equals(this.status, pet.status)&&
+        Objects.equals(this.additionalProperties, pet.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(id, category, name, photoUrls, tags, status, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class Pet {\n");
+    sb.append("    id: ").append(toIndentedString(id)).append("\n");
+    sb.append("    category: ").append(toIndentedString(category)).append("\n");
+    sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    photoUrls: ").append(toIndentedString(photoUrls)).append("\n");
+    sb.append("    tags: ").append(toIndentedString(tags)).append("\n");
+    sb.append("    status: ").append(toIndentedString(status)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>();
+    openapiFields.add("id");
+    openapiFields.add("category");
+    openapiFields.add("name");
+    openapiFields.add("photoUrls");
+    openapiFields.add("tags");
+    openapiFields.add("status");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+    openapiRequiredFields.add("name");
+    openapiRequiredFields.add("photoUrls");
+  }
+
+ /**
+  * Validates the JSON Object and throws an exception if issues found
+  *
+  * @param jsonObj JSON Object
+  * @throws IOException if the JSON Object is invalid with respect to Pet
+  */
+  public static void validateJsonObject(JsonObject jsonObj) throws IOException {
+      if (jsonObj == null) {
+        if (!Pet.openapiRequiredFields.isEmpty()) { // has required fields but JSON object is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in Pet is not found in the empty JSON string", Pet.openapiRequiredFields.toString()));
+        }
+      }
+
+      // check to make sure all required properties/fields are present in the JSON string
+      for (String requiredField : Pet.openapiRequiredFields) {
+        if (jsonObj.get(requiredField) == null) {
+          throw new IllegalArgumentException(String.format("The required field `%s` is not found in the JSON string: %s", requiredField, jsonObj.toString()));
+        }
+      }
+      // validate the optional field `category`
+      if (jsonObj.get("category") != null && !jsonObj.get("category").isJsonNull()) {
+        Category.validateJsonObject(jsonObj.getAsJsonObject("category"));
+      }
+      if (!jsonObj.get("name").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `name` to be a primitive type in the JSON string but got `%s`", jsonObj.get("name").toString()));
+      }
+      // ensure the required json array is present
+      if (jsonObj.get("photoUrls") == null) {
+        throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
+      } else if (!jsonObj.get("photoUrls").isJsonArray()) {
+        throw new IllegalArgumentException(String.format("Expected the field `photoUrls` to be an array in the JSON string but got `%s`", jsonObj.get("photoUrls").toString()));
+      }
+      if (jsonObj.get("tags") != null && !jsonObj.get("tags").isJsonNull()) {
+        JsonArray jsonArraytags = jsonObj.getAsJsonArray("tags");
+        if (jsonArraytags != null) {
+          // ensure the json data is an array
+          if (!jsonObj.get("tags").isJsonArray()) {
+            throw new IllegalArgumentException(String.format("Expected the field `tags` to be an array in the JSON string but got `%s`", jsonObj.get("tags").toString()));
+          }
+
+          // validate the optional field `tags` (array)
+          for (int i = 0; i < jsonArraytags.size(); i++) {
+            Tag.validateJsonObject(jsonArraytags.get(i).getAsJsonObject());
+          };
+        }
+      }
+      if ((jsonObj.get("status") != null && !jsonObj.get("status").isJsonNull()) && !jsonObj.get("status").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `status` to be a primitive type in the JSON string but got `%s`", jsonObj.get("status").toString()));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!Pet.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'Pet' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<Pet> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(Pet.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<Pet>() {
+           @Override
+           public void write(JsonWriter out, Pet value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public Pet read(JsonReader in) throws IOException {
+             JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
+             validateJsonObject(jsonObj);
+             // store additional fields in the deserialized instance
+             Pet instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+ /**
+  * Create an instance of Pet given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of Pet
+  * @throws IOException if the JSON string is invalid with respect to Pet
+  */
+  public static Pet fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, Pet.class);
+  }
+
+ /**
+  * Convert an instance of Pet to an JSON string
+  *
+  * @return JSON string
+  */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases1.java

@@ -0,0 +1,545 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import java.util.Arrays;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import org.openapitools.client.model.Category;
+import org.openapitools.client.model.Tag;
+import org.openapitools.jackson.nullable.JsonNullable;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * A pet for sale in the pet store
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class PetWithRequiredNullableCases1 {
+  public static final String SERIALIZED_NAME_ID = "id";
+  @SerializedName(SERIALIZED_NAME_ID)
+  private Long id;
+
+  public static final String SERIALIZED_NAME_CATEGORY = "category";
+  @SerializedName(SERIALIZED_NAME_CATEGORY)
+  private Category category;
+
+  public static final String SERIALIZED_NAME_NAME = "name";
+  @SerializedName(SERIALIZED_NAME_NAME)
+  private String name;
+
+  public static final String SERIALIZED_NAME_PHOTO_URLS = "photoUrls";
+  @SerializedName(SERIALIZED_NAME_PHOTO_URLS)
+  private List<String> photoUrls;
+
+  public static final String SERIALIZED_NAME_TAGS = "tags";
+  @SerializedName(SERIALIZED_NAME_TAGS)
+  private List<Tag> tags;
+
+  /**
+   * pet status in the store
+   */
+  @JsonAdapter(StatusEnum.Adapter.class)
+  public enum StatusEnum {
+    AVAILABLE("available"),
+    
+    PENDING("pending"),
+    
+    SOLD("sold");
+
+    private String value;
+
+    StatusEnum(String value) {
+      this.value = value;
+    }
+
+    public String getValue() {
+      return value;
+    }
+
+    @Override
+    public String toString() {
+      return String.valueOf(value);
+    }
+
+    public static StatusEnum fromValue(String value) {
+      for (StatusEnum b : StatusEnum.values()) {
+        if (b.value.equals(value)) {
+          return b;
+        }
+      }
+      return null;
+    }
+
+    public static class Adapter extends TypeAdapter<StatusEnum> {
+      @Override
+      public void write(final JsonWriter jsonWriter, final StatusEnum enumeration) throws IOException {
+        jsonWriter.value(enumeration.getValue());
+      }
+
+      @Override
+      public StatusEnum read(final JsonReader jsonReader) throws IOException {
+        String value =  jsonReader.nextString();
+        return StatusEnum.fromValue(value);
+      }
+    }
+  }
+
+  public static final String SERIALIZED_NAME_STATUS = "status";
+  @Deprecated
+  @SerializedName(SERIALIZED_NAME_STATUS)
+  private StatusEnum status;
+
+  public PetWithRequiredNullableCases1() {
+  }
+
+  public PetWithRequiredNullableCases1 id(Long id) {
+    
+    this.id = id;
+    return this;
+  }
+
+   /**
+   * Get id
+   * @return id
+  **/
+  @javax.annotation.Nullable
+  public Long getId() {
+    return id;
+  }
+
+
+  public void setId(Long id) {
+    this.id = id;
+  }
+
+
+  public PetWithRequiredNullableCases1 category(Category category) {
+    
+    this.category = category;
+    return this;
+  }
+
+   /**
+   * Get category
+   * @return category
+  **/
+  @javax.annotation.Nullable
+  public Category getCategory() {
+    return category;
+  }
+
+
+  public void setCategory(Category category) {
+    this.category = category;
+  }
+
+
+  public PetWithRequiredNullableCases1 name(String name) {
+    
+    this.name = name;
+    return this;
+  }
+
+   /**
+   * Get name
+   * @return name
+  **/
+  @javax.annotation.Nullable
+  public String getName() {
+    return name;
+  }
+
+
+  public void setName(String name) {
+    this.name = name;
+  }
+
+
+  public PetWithRequiredNullableCases1 photoUrls(List<String> photoUrls) {
+    
+    this.photoUrls = photoUrls;
+    return this;
+  }
+
+  public PetWithRequiredNullableCases1 addPhotoUrlsItem(String photoUrlsItem) {
+    if (this.photoUrls == null) {
+      this.photoUrls = new ArrayList<>();
+    }
+    this.photoUrls.add(photoUrlsItem);
+    return this;
+  }
+
+   /**
+   * Get photoUrls
+   * @return photoUrls
+  **/
+  @javax.annotation.Nullable
+  public List<String> getPhotoUrls() {
+    return photoUrls;
+  }
+
+
+  public void setPhotoUrls(List<String> photoUrls) {
+    this.photoUrls = photoUrls;
+  }
+
+
+  public PetWithRequiredNullableCases1 tags(List<Tag> tags) {
+    
+    this.tags = tags;
+    return this;
+  }
+
+  public PetWithRequiredNullableCases1 addTagsItem(Tag tagsItem) {
+    if (this.tags == null) {
+      this.tags = new ArrayList<>();
+    }
+    this.tags.add(tagsItem);
+    return this;
+  }
+
+   /**
+   * Get tags
+   * @return tags
+  **/
+  @javax.annotation.Nullable
+  public List<Tag> getTags() {
+    return tags;
+  }
+
+
+  public void setTags(List<Tag> tags) {
+    this.tags = tags;
+  }
+
+
+  @Deprecated
+  public PetWithRequiredNullableCases1 status(StatusEnum status) {
+    
+    this.status = status;
+    return this;
+  }
+
+   /**
+   * pet status in the store
+   * @return status
+   * @deprecated
+  **/
+  @Deprecated
+  @javax.annotation.Nullable
+  public StatusEnum getStatus() {
+    return status;
+  }
+
+
+  @Deprecated
+  public void setStatus(StatusEnum status) {
+    this.status = status;
+  }
+
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the PetWithRequiredNullableCases1 instance itself
+   */
+  public PetWithRequiredNullableCases1 putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    PetWithRequiredNullableCases1 petWithRequiredNullableCases1 = (PetWithRequiredNullableCases1) o;
+    return Objects.equals(this.id, petWithRequiredNullableCases1.id) &&
+        Objects.equals(this.category, petWithRequiredNullableCases1.category) &&
+        Objects.equals(this.name, petWithRequiredNullableCases1.name) &&
+        Objects.equals(this.photoUrls, petWithRequiredNullableCases1.photoUrls) &&
+        Objects.equals(this.tags, petWithRequiredNullableCases1.tags) &&
+        Objects.equals(this.status, petWithRequiredNullableCases1.status)&&
+        Objects.equals(this.additionalProperties, petWithRequiredNullableCases1.additionalProperties);
+  }
+
+  private static <T> boolean equalsNullable(JsonNullable<T> a, JsonNullable<T> b) {
+    return a == b || (a != null && b != null && a.isPresent() && b.isPresent() && Objects.deepEquals(a.get(), b.get()));
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(id, category, name, photoUrls, tags, status, additionalProperties);
+  }
+
+  private static <T> int hashCodeNullable(JsonNullable<T> a) {
+    if (a == null) {
+      return 1;
+    }
+    return a.isPresent() ? Arrays.deepHashCode(new Object[]{a.get()}) : 31;
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class PetWithRequiredNullableCases1 {\n");
+    sb.append("    id: ").append(toIndentedString(id)).append("\n");
+    sb.append("    category: ").append(toIndentedString(category)).append("\n");
+    sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    photoUrls: ").append(toIndentedString(photoUrls)).append("\n");
+    sb.append("    tags: ").append(toIndentedString(tags)).append("\n");
+    sb.append("    status: ").append(toIndentedString(status)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>();
+    openapiFields.add("id");
+    openapiFields.add("category");
+    openapiFields.add("name");
+    openapiFields.add("photoUrls");
+    openapiFields.add("tags");
+    openapiFields.add("status");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+    openapiRequiredFields.add("name");
+    openapiRequiredFields.add("photoUrls");
+  }
+
+ /**
+  * Validates the JSON Object and throws an exception if issues found
+  *
+  * @param jsonObj JSON Object
+  * @throws IOException if the JSON Object is invalid with respect to PetWithRequiredNullableCases1
+  */
+  public static void validateJsonObject(JsonObject jsonObj) throws IOException {
+      if (jsonObj == null) {
+        if (!PetWithRequiredNullableCases1.openapiRequiredFields.isEmpty()) { // has required fields but JSON object is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in PetWithRequiredNullableCases1 is not found in the empty JSON string", PetWithRequiredNullableCases1.openapiRequiredFields.toString()));
+        }
+      }
+
+      // check to make sure all required properties/fields are present in the JSON string
+      for (String requiredField : PetWithRequiredNullableCases1.openapiRequiredFields) {
+        if (jsonObj.get(requiredField) == null) {
+          throw new IllegalArgumentException(String.format("The required field `%s` is not found in the JSON string: %s", requiredField, jsonObj.toString()));
+        }
+      }
+      // validate the optional field `category`
+      if (jsonObj.get("category") != null && !jsonObj.get("category").isJsonNull()) {
+        Category.validateJsonObject(jsonObj.getAsJsonObject("category"));
+      }
+      if ((jsonObj.get("name") != null && !jsonObj.get("name").isJsonNull()) && !jsonObj.get("name").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `name` to be a primitive type in the JSON string but got `%s`", jsonObj.get("name").toString()));
+      }
+      // ensure the required json array is present
+      if (jsonObj.get("photoUrls") == null) {
+        throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
+      } else if (!jsonObj.get("photoUrls").isJsonArray()) {
+        throw new IllegalArgumentException(String.format("Expected the field `photoUrls` to be an array in the JSON string but got `%s`", jsonObj.get("photoUrls").toString()));
+      }
+      if (jsonObj.get("tags") != null && !jsonObj.get("tags").isJsonNull()) {
+        JsonArray jsonArraytags = jsonObj.getAsJsonArray("tags");
+        if (jsonArraytags != null) {
+          // ensure the json data is an array
+          if (!jsonObj.get("tags").isJsonArray()) {
+            throw new IllegalArgumentException(String.format("Expected the field `tags` to be an array in the JSON string but got `%s`", jsonObj.get("tags").toString()));
+          }
+
+          // validate the optional field `tags` (array)
+          for (int i = 0; i < jsonArraytags.size(); i++) {
+            Tag.validateJsonObject(jsonArraytags.get(i).getAsJsonObject());
+          };
+        }
+      }
+      if ((jsonObj.get("status") != null && !jsonObj.get("status").isJsonNull()) && !jsonObj.get("status").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `status` to be a primitive type in the JSON string but got `%s`", jsonObj.get("status").toString()));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!PetWithRequiredNullableCases1.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'PetWithRequiredNullableCases1' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<PetWithRequiredNullableCases1> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(PetWithRequiredNullableCases1.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<PetWithRequiredNullableCases1>() {
+           @Override
+           public void write(JsonWriter out, PetWithRequiredNullableCases1 value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public PetWithRequiredNullableCases1 read(JsonReader in) throws IOException {
+             JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
+             validateJsonObject(jsonObj);
+             // store additional fields in the deserialized instance
+             PetWithRequiredNullableCases1 instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+ /**
+  * Create an instance of PetWithRequiredNullableCases1 given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of PetWithRequiredNullableCases1
+  * @throws IOException if the JSON string is invalid with respect to PetWithRequiredNullableCases1
+  */
+  public static PetWithRequiredNullableCases1 fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, PetWithRequiredNullableCases1.class);
+  }
+
+ /**
+  * Convert an instance of PetWithRequiredNullableCases1 to an JSON string
+  *
+  * @return JSON string
+  */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/PetWithRequiredNullableCases2.java

@@ -0,0 +1,533 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import java.util.Arrays;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import org.openapitools.client.model.Category;
+import org.openapitools.client.model.Tag;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * A pet for sale in the pet store
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class PetWithRequiredNullableCases2 {
+  public static final String SERIALIZED_NAME_ID = "id";
+  @SerializedName(SERIALIZED_NAME_ID)
+  private Long id;
+
+  public static final String SERIALIZED_NAME_CATEGORY = "category";
+  @SerializedName(SERIALIZED_NAME_CATEGORY)
+  private Category category;
+
+  public static final String SERIALIZED_NAME_NAME = "name";
+  @SerializedName(SERIALIZED_NAME_NAME)
+  private String name;
+
+  public static final String SERIALIZED_NAME_PHOTO_URLS = "photoUrls";
+  @SerializedName(SERIALIZED_NAME_PHOTO_URLS)
+  private List<String> photoUrls = new ArrayList<>();
+
+  public static final String SERIALIZED_NAME_TAGS = "tags";
+  @SerializedName(SERIALIZED_NAME_TAGS)
+  private List<Tag> tags;
+
+  /**
+   * pet status in the store
+   */
+  @JsonAdapter(StatusEnum.Adapter.class)
+  public enum StatusEnum {
+    AVAILABLE("available"),
+    
+    PENDING("pending"),
+    
+    SOLD("sold");
+
+    private String value;
+
+    StatusEnum(String value) {
+      this.value = value;
+    }
+
+    public String getValue() {
+      return value;
+    }
+
+    @Override
+    public String toString() {
+      return String.valueOf(value);
+    }
+
+    public static StatusEnum fromValue(String value) {
+      for (StatusEnum b : StatusEnum.values()) {
+        if (b.value.equals(value)) {
+          return b;
+        }
+      }
+      throw new IllegalArgumentException("Unexpected value '" + value + "'");
+    }
+
+    public static class Adapter extends TypeAdapter<StatusEnum> {
+      @Override
+      public void write(final JsonWriter jsonWriter, final StatusEnum enumeration) throws IOException {
+        jsonWriter.value(enumeration.getValue());
+      }
+
+      @Override
+      public StatusEnum read(final JsonReader jsonReader) throws IOException {
+        String value =  jsonReader.nextString();
+        return StatusEnum.fromValue(value);
+      }
+    }
+  }
+
+  public static final String SERIALIZED_NAME_STATUS = "status";
+  @Deprecated
+  @SerializedName(SERIALIZED_NAME_STATUS)
+  private StatusEnum status;
+
+  public PetWithRequiredNullableCases2() {
+  }
+
+  public PetWithRequiredNullableCases2 id(Long id) {
+    
+    this.id = id;
+    return this;
+  }
+
+   /**
+   * Get id
+   * @return id
+  **/
+  @javax.annotation.Nullable
+  public Long getId() {
+    return id;
+  }
+
+
+  public void setId(Long id) {
+    this.id = id;
+  }
+
+
+  public PetWithRequiredNullableCases2 category(Category category) {
+    
+    this.category = category;
+    return this;
+  }
+
+   /**
+   * Get category
+   * @return category
+  **/
+  @javax.annotation.Nullable
+  public Category getCategory() {
+    return category;
+  }
+
+
+  public void setCategory(Category category) {
+    this.category = category;
+  }
+
+
+  public PetWithRequiredNullableCases2 name(String name) {
+    
+    this.name = name;
+    return this;
+  }
+
+   /**
+   * Get name
+   * @return name
+  **/
+  @javax.annotation.Nonnull
+  public String getName() {
+    return name;
+  }
+
+
+  public void setName(String name) {
+    this.name = name;
+  }
+
+
+  public PetWithRequiredNullableCases2 photoUrls(List<String> photoUrls) {
+    
+    this.photoUrls = photoUrls;
+    return this;
+  }
+
+  public PetWithRequiredNullableCases2 addPhotoUrlsItem(String photoUrlsItem) {
+    if (this.photoUrls == null) {
+      this.photoUrls = new ArrayList<>();
+    }
+    this.photoUrls.add(photoUrlsItem);
+    return this;
+  }
+
+   /**
+   * Get photoUrls
+   * @return photoUrls
+  **/
+  @javax.annotation.Nonnull
+  public List<String> getPhotoUrls() {
+    return photoUrls;
+  }
+
+
+  public void setPhotoUrls(List<String> photoUrls) {
+    this.photoUrls = photoUrls;
+  }
+
+
+  public PetWithRequiredNullableCases2 tags(List<Tag> tags) {
+    
+    this.tags = tags;
+    return this;
+  }
+
+  public PetWithRequiredNullableCases2 addTagsItem(Tag tagsItem) {
+    if (this.tags == null) {
+      this.tags = new ArrayList<>();
+    }
+    this.tags.add(tagsItem);
+    return this;
+  }
+
+   /**
+   * Get tags
+   * @return tags
+  **/
+  @javax.annotation.Nullable
+  public List<Tag> getTags() {
+    return tags;
+  }
+
+
+  public void setTags(List<Tag> tags) {
+    this.tags = tags;
+  }
+
+
+  @Deprecated
+  public PetWithRequiredNullableCases2 status(StatusEnum status) {
+    
+    this.status = status;
+    return this;
+  }
+
+   /**
+   * pet status in the store
+   * @return status
+   * @deprecated
+  **/
+  @Deprecated
+  @javax.annotation.Nullable
+  public StatusEnum getStatus() {
+    return status;
+  }
+
+
+  @Deprecated
+  public void setStatus(StatusEnum status) {
+    this.status = status;
+  }
+
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the PetWithRequiredNullableCases2 instance itself
+   */
+  public PetWithRequiredNullableCases2 putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    PetWithRequiredNullableCases2 petWithRequiredNullableCases2 = (PetWithRequiredNullableCases2) o;
+    return Objects.equals(this.id, petWithRequiredNullableCases2.id) &&
+        Objects.equals(this.category, petWithRequiredNullableCases2.category) &&
+        Objects.equals(this.name, petWithRequiredNullableCases2.name) &&
+        Objects.equals(this.photoUrls, petWithRequiredNullableCases2.photoUrls) &&
+        Objects.equals(this.tags, petWithRequiredNullableCases2.tags) &&
+        Objects.equals(this.status, petWithRequiredNullableCases2.status)&&
+        Objects.equals(this.additionalProperties, petWithRequiredNullableCases2.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(id, category, name, photoUrls, tags, status, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class PetWithRequiredNullableCases2 {\n");
+    sb.append("    id: ").append(toIndentedString(id)).append("\n");
+    sb.append("    category: ").append(toIndentedString(category)).append("\n");
+    sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    photoUrls: ").append(toIndentedString(photoUrls)).append("\n");
+    sb.append("    tags: ").append(toIndentedString(tags)).append("\n");
+    sb.append("    status: ").append(toIndentedString(status)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>();
+    openapiFields.add("id");
+    openapiFields.add("category");
+    openapiFields.add("name");
+    openapiFields.add("photoUrls");
+    openapiFields.add("tags");
+    openapiFields.add("status");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+    openapiRequiredFields.add("name");
+    openapiRequiredFields.add("photoUrls");
+  }
+
+ /**
+  * Validates the JSON Object and throws an exception if issues found
+  *
+  * @param jsonObj JSON Object
+  * @throws IOException if the JSON Object is invalid with respect to PetWithRequiredNullableCases2
+  */
+  public static void validateJsonObject(JsonObject jsonObj) throws IOException {
+      if (jsonObj == null) {
+        if (!PetWithRequiredNullableCases2.openapiRequiredFields.isEmpty()) { // has required fields but JSON object is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in PetWithRequiredNullableCases2 is not found in the empty JSON string", PetWithRequiredNullableCases2.openapiRequiredFields.toString()));
+        }
+      }
+
+      // check to make sure all required properties/fields are present in the JSON string
+      for (String requiredField : PetWithRequiredNullableCases2.openapiRequiredFields) {
+        if (jsonObj.get(requiredField) == null) {
+          throw new IllegalArgumentException(String.format("The required field `%s` is not found in the JSON string: %s", requiredField, jsonObj.toString()));
+        }
+      }
+      // validate the optional field `category`
+      if (jsonObj.get("category") != null && !jsonObj.get("category").isJsonNull()) {
+        Category.validateJsonObject(jsonObj.getAsJsonObject("category"));
+      }
+      if (!jsonObj.get("name").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `name` to be a primitive type in the JSON string but got `%s`", jsonObj.get("name").toString()));
+      }
+      // ensure the required json array is present
+      if (jsonObj.get("photoUrls") == null) {
+        throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
+      } else if (!jsonObj.get("photoUrls").isJsonArray()) {
+        throw new IllegalArgumentException(String.format("Expected the field `photoUrls` to be an array in the JSON string but got `%s`", jsonObj.get("photoUrls").toString()));
+      }
+      if (jsonObj.get("tags") != null && !jsonObj.get("tags").isJsonNull()) {
+        JsonArray jsonArraytags = jsonObj.getAsJsonArray("tags");
+        if (jsonArraytags != null) {
+          // ensure the json data is an array
+          if (!jsonObj.get("tags").isJsonArray()) {
+            throw new IllegalArgumentException(String.format("Expected the field `tags` to be an array in the JSON string but got `%s`", jsonObj.get("tags").toString()));
+          }
+
+          // validate the optional field `tags` (array)
+          for (int i = 0; i < jsonArraytags.size(); i++) {
+            Tag.validateJsonObject(jsonArraytags.get(i).getAsJsonObject());
+          };
+        }
+      }
+      if ((jsonObj.get("status") != null && !jsonObj.get("status").isJsonNull()) && !jsonObj.get("status").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `status` to be a primitive type in the JSON string but got `%s`", jsonObj.get("status").toString()));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!PetWithRequiredNullableCases2.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'PetWithRequiredNullableCases2' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<PetWithRequiredNullableCases2> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(PetWithRequiredNullableCases2.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<PetWithRequiredNullableCases2>() {
+           @Override
+           public void write(JsonWriter out, PetWithRequiredNullableCases2 value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public PetWithRequiredNullableCases2 read(JsonReader in) throws IOException {
+             JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
+             validateJsonObject(jsonObj);
+             // store additional fields in the deserialized instance
+             PetWithRequiredNullableCases2 instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+ /**
+  * Create an instance of PetWithRequiredNullableCases2 given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of PetWithRequiredNullableCases2
+  * @throws IOException if the JSON string is invalid with respect to PetWithRequiredNullableCases2
+  */
+  public static PetWithRequiredNullableCases2 fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, PetWithRequiredNullableCases2.class);
+  }
+
+ /**
+  * Convert an instance of PetWithRequiredNullableCases2 to an JSON string
+  *
+  * @return JSON string
+  */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/Tag.java

@@ -0,0 +1,311 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import java.util.Arrays;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * A tag for a pet
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class Tag {
+  public static final String SERIALIZED_NAME_ID = "id";
+  @SerializedName(SERIALIZED_NAME_ID)
+  private Long id;
+
+  public static final String SERIALIZED_NAME_NAME = "name";
+  @SerializedName(SERIALIZED_NAME_NAME)
+  private String name;
+
+  public Tag() {
+  }
+
+  public Tag id(Long id) {
+    
+    this.id = id;
+    return this;
+  }
+
+   /**
+   * Get id
+   * @return id
+  **/
+  @javax.annotation.Nullable
+  public Long getId() {
+    return id;
+  }
+
+
+  public void setId(Long id) {
+    this.id = id;
+  }
+
+
+  public Tag name(String name) {
+    
+    this.name = name;
+    return this;
+  }
+
+   /**
+   * Get name
+   * @return name
+  **/
+  @javax.annotation.Nullable
+  public String getName() {
+    return name;
+  }
+
+
+  public void setName(String name) {
+    this.name = name;
+  }
+
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the Tag instance itself
+   */
+  public Tag putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    Tag tag = (Tag) o;
+    return Objects.equals(this.id, tag.id) &&
+        Objects.equals(this.name, tag.name)&&
+        Objects.equals(this.additionalProperties, tag.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(id, name, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class Tag {\n");
+    sb.append("    id: ").append(toIndentedString(id)).append("\n");
+    sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>();
+    openapiFields.add("id");
+    openapiFields.add("name");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+  }
+
+ /**
+  * Validates the JSON Object and throws an exception if issues found
+  *
+  * @param jsonObj JSON Object
+  * @throws IOException if the JSON Object is invalid with respect to Tag
+  */
+  public static void validateJsonObject(JsonObject jsonObj) throws IOException {
+      if (jsonObj == null) {
+        if (!Tag.openapiRequiredFields.isEmpty()) { // has required fields but JSON object is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in Tag is not found in the empty JSON string", Tag.openapiRequiredFields.toString()));
+        }
+      }
+      if ((jsonObj.get("name") != null && !jsonObj.get("name").isJsonNull()) && !jsonObj.get("name").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `name` to be a primitive type in the JSON string but got `%s`", jsonObj.get("name").toString()));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!Tag.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'Tag' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<Tag> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(Tag.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<Tag>() {
+           @Override
+           public void write(JsonWriter out, Tag value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public Tag read(JsonReader in) throws IOException {
+             JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
+             validateJsonObject(jsonObj);
+             // store additional fields in the deserialized instance
+             Tag instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+ /**
+  * Create an instance of Tag given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of Tag
+  * @throws IOException if the JSON string is invalid with respect to Tag
+  */
+  public static Tag fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, Tag.class);
+  }
+
+ /**
+  * Convert an instance of Tag to an JSON string
+  *
+  * @return JSON string
+  */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+

samples/client/petstore/java/okhttp-gson-nullable-required/src/main/java/org/openapitools/client/model/User.java

@@ -0,0 +1,494 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import java.util.Arrays;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * A User who is purchasing from the pet store
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen")
+public class User {
+  public static final String SERIALIZED_NAME_ID = "id";
+  @SerializedName(SERIALIZED_NAME_ID)
+  private Long id;
+
+  public static final String SERIALIZED_NAME_USERNAME = "username";
+  @SerializedName(SERIALIZED_NAME_USERNAME)
+  private String username;
+
+  public static final String SERIALIZED_NAME_FIRST_NAME = "firstName";
+  @SerializedName(SERIALIZED_NAME_FIRST_NAME)
+  private String firstName;
+
+  public static final String SERIALIZED_NAME_LAST_NAME = "lastName";
+  @SerializedName(SERIALIZED_NAME_LAST_NAME)
+  private String lastName;
+
+  public static final String SERIALIZED_NAME_EMAIL = "email";
+  @SerializedName(SERIALIZED_NAME_EMAIL)
+  private String email;
+
+  public static final String SERIALIZED_NAME_PASSWORD = "password";
+  @SerializedName(SERIALIZED_NAME_PASSWORD)
+  private String password;
+
+  public static final String SERIALIZED_NAME_PHONE = "phone";
+  @SerializedName(SERIALIZED_NAME_PHONE)
+  private String phone;
+
+  public static final String SERIALIZED_NAME_USER_STATUS = "userStatus";
+  @SerializedName(SERIALIZED_NAME_USER_STATUS)
+  private Integer userStatus;
+
+  public User() {
+  }
+
+  public User id(Long id) {
+    
+    this.id = id;
+    return this;
+  }
+
+   /**
+   * Get id
+   * @return id
+  **/
+  @javax.annotation.Nullable
+  public Long getId() {
+    return id;
+  }
+
+
+  public void setId(Long id) {
+    this.id = id;
+  }
+
+
+  public User username(String username) {
+    
+    this.username = username;
+    return this;
+  }
+
+   /**
+   * Get username
+   * @return username
+  **/
+  @javax.annotation.Nullable
+  public String getUsername() {
+    return username;
+  }
+
+
+  public void setUsername(String username) {
+    this.username = username;
+  }
+
+
+  public User firstName(String firstName) {
+    
+    this.firstName = firstName;
+    return this;
+  }
+
+   /**
+   * Get firstName
+   * @return firstName
+  **/
+  @javax.annotation.Nullable
+  public String getFirstName() {
+    return firstName;
+  }
+
+
+  public void setFirstName(String firstName) {
+    this.firstName = firstName;
+  }
+
+
+  public User lastName(String lastName) {
+    
+    this.lastName = lastName;
+    return this;
+  }
+
+   /**
+   * Get lastName
+   * @return lastName
+  **/
+  @javax.annotation.Nullable
+  public String getLastName() {
+    return lastName;
+  }
+
+
+  public void setLastName(String lastName) {
+    this.lastName = lastName;
+  }
+
+
+  public User email(String email) {
+    
+    this.email = email;
+    return this;
+  }
+
+   /**
+   * Get email
+   * @return email
+  **/
+  @javax.annotation.Nullable
+  public String getEmail() {
+    return email;
+  }
+
+
+  public void setEmail(String email) {
+    this.email = email;
+  }
+
+
+  public User password(String password) {
+    
+    this.password = password;
+    return this;
+  }
+
+   /**
+   * Get password
+   * @return password
+  **/
+  @javax.annotation.Nullable
+  public String getPassword() {
+    return password;
+  }
+
+
+  public void setPassword(String password) {
+    this.password = password;
+  }
+
+
+  public User phone(String phone) {
+    
+    this.phone = phone;
+    return this;
+  }
+
+   /**
+   * Get phone
+   * @return phone
+  **/
+  @javax.annotation.Nullable
+  public String getPhone() {
+    return phone;
+  }
+
+
+  public void setPhone(String phone) {
+    this.phone = phone;
+  }
+
+
+  public User userStatus(Integer userStatus) {
+    
+    this.userStatus = userStatus;
+    return this;
+  }
+
+   /**
+   * User Status
+   * @return userStatus
+  **/
+  @javax.annotation.Nullable
+  public Integer getUserStatus() {
+    return userStatus;
+  }
+
+
+  public void setUserStatus(Integer userStatus) {
+    this.userStatus = userStatus;
+  }
+
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the User instance itself
+   */
+  public User putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    User user = (User) o;
+    return Objects.equals(this.id, user.id) &&
+        Objects.equals(this.username, user.username) &&
+        Objects.equals(this.firstName, user.firstName) &&
+        Objects.equals(this.lastName, user.lastName) &&
+        Objects.equals(this.email, user.email) &&
+        Objects.equals(this.password, user.password) &&
+        Objects.equals(this.phone, user.phone) &&
+        Objects.equals(this.userStatus, user.userStatus)&&
+        Objects.equals(this.additionalProperties, user.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(id, username, firstName, lastName, email, password, phone, userStatus, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class User {\n");
+    sb.append("    id: ").append(toIndentedString(id)).append("\n");
+    sb.append("    username: ").append(toIndentedString(username)).append("\n");
+    sb.append("    firstName: ").append(toIndentedString(firstName)).append("\n");
+    sb.append("    lastName: ").append(toIndentedString(lastName)).append("\n");
+    sb.append("    email: ").append(toIndentedString(email)).append("\n");
+    sb.append("    password: ").append(toIndentedString(password)).append("\n");
+    sb.append("    phone: ").append(toIndentedString(phone)).append("\n");
+    sb.append("    userStatus: ").append(toIndentedString(userStatus)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>();
+    openapiFields.add("id");
+    openapiFields.add("username");
+    openapiFields.add("firstName");
+    openapiFields.add("lastName");
+    openapiFields.add("email");
+    openapiFields.add("password");
+    openapiFields.add("phone");
+    openapiFields.add("userStatus");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+  }
+
+ /**
+  * Validates the JSON Object and throws an exception if issues found
+  *
+  * @param jsonObj JSON Object
+  * @throws IOException if the JSON Object is invalid with respect to User
+  */
+  public static void validateJsonObject(JsonObject jsonObj) throws IOException {
+      if (jsonObj == null) {
+        if (!User.openapiRequiredFields.isEmpty()) { // has required fields but JSON object is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in User is not found in the empty JSON string", User.openapiRequiredFields.toString()));
+        }
+      }
+      if ((jsonObj.get("username") != null && !jsonObj.get("username").isJsonNull()) && !jsonObj.get("username").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `username` to be a primitive type in the JSON string but got `%s`", jsonObj.get("username").toString()));
+      }
+      if ((jsonObj.get("firstName") != null && !jsonObj.get("firstName").isJsonNull()) && !jsonObj.get("firstName").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `firstName` to be a primitive type in the JSON string but got `%s`", jsonObj.get("firstName").toString()));
+      }
+      if ((jsonObj.get("lastName") != null && !jsonObj.get("lastName").isJsonNull()) && !jsonObj.get("lastName").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `lastName` to be a primitive type in the JSON string but got `%s`", jsonObj.get("lastName").toString()));
+      }
+      if ((jsonObj.get("email") != null && !jsonObj.get("email").isJsonNull()) && !jsonObj.get("email").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `email` to be a primitive type in the JSON string but got `%s`", jsonObj.get("email").toString()));
+      }
+      if ((jsonObj.get("password") != null && !jsonObj.get("password").isJsonNull()) && !jsonObj.get("password").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `password` to be a primitive type in the JSON string but got `%s`", jsonObj.get("password").toString()));
+      }
+      if ((jsonObj.get("phone") != null && !jsonObj.get("phone").isJsonNull()) && !jsonObj.get("phone").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `phone` to be a primitive type in the JSON string but got `%s`", jsonObj.get("phone").toString()));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!User.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'User' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<User> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(User.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<User>() {
+           @Override
+           public void write(JsonWriter out, User value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   obj.add(entry.getKey(), gson.toJsonTree(entry.getValue()).getAsJsonObject());
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public User read(JsonReader in) throws IOException {
+             JsonObject jsonObj = elementAdapter.read(in).getAsJsonObject();
+             validateJsonObject(jsonObj);
+             // store additional fields in the deserialized instance
+             User instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+ /**
+  * Create an instance of User given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of User
+  * @throws IOException if the JSON string is invalid with respect to User
+  */
+  public static User fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, User.class);
+  }
+
+ /**
+  * Convert an instance of User to an JSON string
+  *
+  * @return JSON string
+  */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/api/PetApiTest.java

@@ -0,0 +1,153 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ *
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.api;
+
+import org.openapitools.client.ApiException;
+import java.io.File;
+import org.openapitools.client.model.ModelApiResponse;
+import org.openapitools.client.model.Pet;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * API tests for PetApi
+ */
+@Disabled
+public class PetApiTest {
+
+    private final PetApi api = new PetApi();
+
+    /**
+     * Add a new pet to the store
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void addPetTest() throws ApiException {
+        Pet pet = null;
+        api.addPet(pet);
+        // TODO: test validations
+    }
+
+    /**
+     * Deletes a pet
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void deletePetTest() throws ApiException {
+        Long petId = null;
+        String apiKey = null;
+        api.deletePet(petId, apiKey);
+        // TODO: test validations
+    }
+
+    /**
+     * Finds Pets by status
+     *
+     * Multiple status values can be provided with comma separated strings
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void findPetsByStatusTest() throws ApiException {
+        List<String> status = null;
+        List<Pet> response = api.findPetsByStatus(status);
+        // TODO: test validations
+    }
+
+    /**
+     * Finds Pets by tags
+     *
+     * Multiple tags can be provided with comma separated strings. Use tag1, tag2, tag3 for testing.
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void findPetsByTagsTest() throws ApiException {
+        List<String> tags = null;
+        List<Pet> response = api.findPetsByTags(tags);
+        // TODO: test validations
+    }
+
+    /**
+     * Find pet by ID
+     *
+     * Returns a single pet
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void getPetByIdTest() throws ApiException {
+        Long petId = null;
+        Pet response = api.getPetById(petId);
+        // TODO: test validations
+    }
+
+    /**
+     * Update an existing pet
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void updatePetTest() throws ApiException {
+        Pet pet = null;
+        api.updatePet(pet);
+        // TODO: test validations
+    }
+
+    /**
+     * Updates a pet in the store with form data
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void updatePetWithFormTest() throws ApiException {
+        Long petId = null;
+        String name = null;
+        String status = null;
+        api.updatePetWithForm(petId, name, status);
+        // TODO: test validations
+    }
+
+    /**
+     * uploads an image
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void uploadFileTest() throws ApiException {
+        Long petId = null;
+        String additionalMetadata = null;
+        File _file = null;
+        ModelApiResponse response = api.uploadFile(petId, additionalMetadata, _file);
+        // TODO: test validations
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/api/StoreApiTest.java

@@ -0,0 +1,89 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ *
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.api;
+
+import org.openapitools.client.ApiException;
+import org.openapitools.client.model.Order;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * API tests for StoreApi
+ */
+@Disabled
+public class StoreApiTest {
+
+    private final StoreApi api = new StoreApi();
+
+    /**
+     * Delete purchase order by ID
+     *
+     * For valid response try integer IDs with value < 1000. Anything above 1000 or nonintegers will generate API errors
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void deleteOrderTest() throws ApiException {
+        String orderId = null;
+        api.deleteOrder(orderId);
+        // TODO: test validations
+    }
+
+    /**
+     * Returns pet inventories by status
+     *
+     * Returns a map of status codes to quantities
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void getInventoryTest() throws ApiException {
+        Map<String, Integer> response = api.getInventory();
+        // TODO: test validations
+    }
+
+    /**
+     * Find purchase order by ID
+     *
+     * For valid response try integer IDs with value &lt;&#x3D; 5 or &gt; 10. Other values will generate exceptions
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void getOrderByIdTest() throws ApiException {
+        Long orderId = null;
+        Order response = api.getOrderById(orderId);
+        // TODO: test validations
+    }
+
+    /**
+     * Place an order for a pet
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void placeOrderTest() throws ApiException {
+        Order order = null;
+        Order response = api.placeOrder(order);
+        // TODO: test validations
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/api/UserApiTest.java

@@ -0,0 +1,148 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ *
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.api;
+
+import org.openapitools.client.ApiException;
+import java.time.OffsetDateTime;
+import org.openapitools.client.model.User;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+import java.util.ArrayList;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * API tests for UserApi
+ */
+@Disabled
+public class UserApiTest {
+
+    private final UserApi api = new UserApi();
+
+    /**
+     * Create user
+     *
+     * This can only be done by the logged in user.
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void createUserTest() throws ApiException {
+        User user = null;
+        api.createUser(user);
+        // TODO: test validations
+    }
+
+    /**
+     * Creates list of users with given input array
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void createUsersWithArrayInputTest() throws ApiException {
+        List<User> user = null;
+        api.createUsersWithArrayInput(user);
+        // TODO: test validations
+    }
+
+    /**
+     * Creates list of users with given input array
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void createUsersWithListInputTest() throws ApiException {
+        List<User> user = null;
+        api.createUsersWithListInput(user);
+        // TODO: test validations
+    }
+
+    /**
+     * Delete user
+     *
+     * This can only be done by the logged in user.
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void deleteUserTest() throws ApiException {
+        String username = null;
+        api.deleteUser(username);
+        // TODO: test validations
+    }
+
+    /**
+     * Get user by user name
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void getUserByNameTest() throws ApiException {
+        String username = null;
+        User response = api.getUserByName(username);
+        // TODO: test validations
+    }
+
+    /**
+     * Logs user into the system
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void loginUserTest() throws ApiException {
+        String username = null;
+        String password = null;
+        String response = api.loginUser(username, password);
+        // TODO: test validations
+    }
+
+    /**
+     * Logs out current logged in user session
+     *
+     *
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void logoutUserTest() throws ApiException {
+        api.logoutUser();
+        // TODO: test validations
+    }
+
+    /**
+     * Updated user
+     *
+     * This can only be done by the logged in user.
+     *
+     * @throws ApiException if the Api call fails
+     */
+    @Test
+    public void updateUserTest() throws ApiException {
+        String username = null;
+        User user = null;
+        api.updateUser(username, user);
+        // TODO: test validations
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/model/CategoryTest.java

@@ -0,0 +1,56 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ *
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for Category
+ */
+public class CategoryTest {
+    private final Category model = new Category();
+
+    /**
+     * Model tests for Category
+     */
+    @Test
+    public void testCategory() {
+        // TODO: test Category
+    }
+
+    /**
+     * Test the property 'id'
+     */
+    @Test
+    public void idTest() {
+        // TODO: test id
+    }
+
+    /**
+     * Test the property 'name'
+     */
+    @Test
+    public void nameTest() {
+        // TODO: test name
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/model/ModelApiResponseTest.java

@@ -0,0 +1,64 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ *
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for ModelApiResponse
+ */
+public class ModelApiResponseTest {
+    private final ModelApiResponse model = new ModelApiResponse();
+
+    /**
+     * Model tests for ModelApiResponse
+     */
+    @Test
+    public void testModelApiResponse() {
+        // TODO: test ModelApiResponse
+    }
+
+    /**
+     * Test the property 'code'
+     */
+    @Test
+    public void codeTest() {
+        // TODO: test code
+    }
+
+    /**
+     * Test the property 'type'
+     */
+    @Test
+    public void typeTest() {
+        // TODO: test type
+    }
+
+    /**
+     * Test the property 'message'
+     */
+    @Test
+    public void messageTest() {
+        // TODO: test message
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/model/OrderTest.java

@@ -0,0 +1,89 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ *
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.time.OffsetDateTime;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for Order
+ */
+public class OrderTest {
+    private final Order model = new Order();
+
+    /**
+     * Model tests for Order
+     */
+    @Test
+    public void testOrder() {
+        // TODO: test Order
+    }
+
+    /**
+     * Test the property 'id'
+     */
+    @Test
+    public void idTest() {
+        // TODO: test id
+    }
+
+    /**
+     * Test the property 'petId'
+     */
+    @Test
+    public void petIdTest() {
+        // TODO: test petId
+    }
+
+    /**
+     * Test the property 'quantity'
+     */
+    @Test
+    public void quantityTest() {
+        // TODO: test quantity
+    }
+
+    /**
+     * Test the property 'shipDate'
+     */
+    @Test
+    public void shipDateTest() {
+        // TODO: test shipDate
+    }
+
+    /**
+     * Test the property 'status'
+     */
+    @Test
+    public void statusTest() {
+        // TODO: test status
+    }
+
+    /**
+     * Test the property 'complete'
+     */
+    @Test
+    public void completeTest() {
+        // TODO: test complete
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/model/PetTest.java

@@ -0,0 +1,92 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ *
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import org.openapitools.client.model.Category;
+import org.openapitools.client.model.Tag;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for Pet
+ */
+public class PetTest {
+    private final Pet model = new Pet();
+
+    /**
+     * Model tests for Pet
+     */
+    @Test
+    public void testPet() {
+        // TODO: test Pet
+    }
+
+    /**
+     * Test the property 'id'
+     */
+    @Test
+    public void idTest() {
+        // TODO: test id
+    }
+
+    /**
+     * Test the property 'category'
+     */
+    @Test
+    public void categoryTest() {
+        // TODO: test category
+    }
+
+    /**
+     * Test the property 'name'
+     */
+    @Test
+    public void nameTest() {
+        // TODO: test name
+    }
+
+    /**
+     * Test the property 'photoUrls'
+     */
+    @Test
+    public void photoUrlsTest() {
+        // TODO: test photoUrls
+    }
+
+    /**
+     * Test the property 'tags'
+     */
+    @Test
+    public void tagsTest() {
+        // TODO: test tags
+    }
+
+    /**
+     * Test the property 'status'
+     */
+    @Test
+    public void statusTest() {
+        // TODO: test status
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/model/PetWithRequiredNullableCases1Test.java

@@ -0,0 +1,93 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import org.openapitools.client.model.Category;
+import org.openapitools.client.model.Tag;
+import org.openapitools.jackson.nullable.JsonNullable;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for PetWithRequiredNullableCases1
+ */
+public class PetWithRequiredNullableCases1Test {
+    private final PetWithRequiredNullableCases1 model = new PetWithRequiredNullableCases1();
+
+    /**
+     * Model tests for PetWithRequiredNullableCases1
+     */
+    @Test
+    public void testPetWithRequiredNullableCases1() {
+        // TODO: test PetWithRequiredNullableCases1
+    }
+
+    /**
+     * Test the property 'id'
+     */
+    @Test
+    public void idTest() {
+        // TODO: test id
+    }
+
+    /**
+     * Test the property 'category'
+     */
+    @Test
+    public void categoryTest() {
+        // TODO: test category
+    }
+
+    /**
+     * Test the property 'name'
+     */
+    @Test
+    public void nameTest() {
+        // TODO: test name
+    }
+
+    /**
+     * Test the property 'photoUrls'
+     */
+    @Test
+    public void photoUrlsTest() {
+        // TODO: test photoUrls
+    }
+
+    /**
+     * Test the property 'tags'
+     */
+    @Test
+    public void tagsTest() {
+        // TODO: test tags
+    }
+
+    /**
+     * Test the property 'status'
+     */
+    @Test
+    public void statusTest() {
+        // TODO: test status
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/model/PetWithRequiredNullableCases2Test.java

@@ -0,0 +1,92 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.List;
+import org.openapitools.client.model.Category;
+import org.openapitools.client.model.Tag;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for PetWithRequiredNullableCases2
+ */
+public class PetWithRequiredNullableCases2Test {
+    private final PetWithRequiredNullableCases2 model = new PetWithRequiredNullableCases2();
+
+    /**
+     * Model tests for PetWithRequiredNullableCases2
+     */
+    @Test
+    public void testPetWithRequiredNullableCases2() {
+        // TODO: test PetWithRequiredNullableCases2
+    }
+
+    /**
+     * Test the property 'id'
+     */
+    @Test
+    public void idTest() {
+        // TODO: test id
+    }
+
+    /**
+     * Test the property 'category'
+     */
+    @Test
+    public void categoryTest() {
+        // TODO: test category
+    }
+
+    /**
+     * Test the property 'name'
+     */
+    @Test
+    public void nameTest() {
+        // TODO: test name
+    }
+
+    /**
+     * Test the property 'photoUrls'
+     */
+    @Test
+    public void photoUrlsTest() {
+        // TODO: test photoUrls
+    }
+
+    /**
+     * Test the property 'tags'
+     */
+    @Test
+    public void tagsTest() {
+        // TODO: test tags
+    }
+
+    /**
+     * Test the property 'status'
+     */
+    @Test
+    public void statusTest() {
+        // TODO: test status
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/model/TagTest.java

@@ -0,0 +1,56 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ *
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for Tag
+ */
+public class TagTest {
+    private final Tag model = new Tag();
+
+    /**
+     * Model tests for Tag
+     */
+    @Test
+    public void testTag() {
+        // TODO: test Tag
+    }
+
+    /**
+     * Test the property 'id'
+     */
+    @Test
+    public void idTest() {
+        // TODO: test id
+    }
+
+    /**
+     * Test the property 'name'
+     */
+    @Test
+    public void nameTest() {
+        // TODO: test name
+    }
+
+}

samples/client/petstore/java/okhttp-gson-nullable-required/src/test/java/org/openapitools/client/model/UserTest.java

@@ -0,0 +1,104 @@
+/*
+ * OpenAPI Petstore
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ *
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for User
+ */
+public class UserTest {
+    private final User model = new User();
+
+    /**
+     * Model tests for User
+     */
+    @Test
+    public void testUser() {
+        // TODO: test User
+    }
+
+    /**
+     * Test the property 'id'
+     */
+    @Test
+    public void idTest() {
+        // TODO: test id
+    }
+
+    /**
+     * Test the property 'username'
+     */
+    @Test
+    public void usernameTest() {
+        // TODO: test username
+    }
+
+    /**
+     * Test the property 'firstName'
+     */
+    @Test
+    public void firstNameTest() {
+        // TODO: test firstName
+    }
+
+    /**
+     * Test the property 'lastName'
+     */
+    @Test
+    public void lastNameTest() {
+        // TODO: test lastName
+    }
+
+    /**
+     * Test the property 'email'
+     */
+    @Test
+    public void emailTest() {
+        // TODO: test email
+    }
+
+    /**
+     * Test the property 'password'
+     */
+    @Test
+    public void passwordTest() {
+        // TODO: test password
+    }
+
+    /**
+     * Test the property 'phone'
+     */
+    @Test
+    public void phoneTest() {
+        // TODO: test phone
+    }
+
+    /**
+     * Test the property 'userStatus'
+     */
+    @Test
+    public void userStatusTest() {
+        // TODO: test userStatus
+    }
+
+}