[java][okhttp-gson-nextgen] better oneOf implementation (#11146)

* add validJsonObject method

* add check for null

* fix list model generation

* fix optional fields validation

* add tests

* fix variable naming

* update tests

* add fromJson in oneOf, add tests

* convert JSON to static

* remove trailing space

* add fromString methods to all models

* add toJson, fix anyOf template

* remove workarounds

* undo changes to tests

* skip file schema test

* add new file

samples/client/petstore/java/feign/src/test/java/org/openapitools/client/model/ModelFileTest.java

@@ -0,0 +1,48 @@
+/*
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * 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.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonTypeName;
+import com.fasterxml.jackson.annotation.JsonValue;
+import io.swagger.annotations.ApiModel;
+import io.swagger.annotations.ApiModelProperty;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for ModelFile
+ */
+class ModelFileTest {
+    private final ModelFile model = new ModelFile();
+
+    /**
+     * Model tests for ModelFile
+     */
+    @Test
+    void testModelFile() {
+        // TODO: test ModelFile
+    }
+
+    /**
+     * Test the property 'sourceURI'
+     */
+    @Test
+    void sourceURITest() {
+        // TODO: test sourceURI
+    }
+
+}

samples/client/petstore/java/feign/src/test/java/org/openapitools/client/model/ModelListTest.java

@@ -0,0 +1,48 @@
+/*
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * 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.fasterxml.jackson.annotation.JsonInclude;
+import com.fasterxml.jackson.annotation.JsonProperty;
+import com.fasterxml.jackson.annotation.JsonCreator;
+import com.fasterxml.jackson.annotation.JsonTypeName;
+import com.fasterxml.jackson.annotation.JsonValue;
+import io.swagger.annotations.ApiModel;
+import io.swagger.annotations.ApiModelProperty;
+import org.junit.jupiter.api.Test;
+
+
+/**
+ * Model tests for ModelList
+ */
+class ModelListTest {
+    private final ModelList model = new ModelList();
+
+    /**
+     * Model tests for ModelList
+     */
+    @Test
+    void testModelList() {
+        // TODO: test ModelList
+    }
+
+    /**
+     * Test the property '_123list'
+     */
+    @Test
+    void _123listTest() {
+        // TODO: test _123list
+    }
+
+}