diff --git a/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/AndroidClientCodegen.java b/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/AndroidClientCodegen.java
index 9a3237d92ca..e65736a63f0 100644
--- a/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/AndroidClientCodegen.java
+++ b/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/AndroidClientCodegen.java
@@ -232,6 +232,8 @@ public class AndroidClientCodegen extends DefaultCodegen implements CodegenConfi
                 (sourceFolder + File.separator + invokerPackage).replace(".", java.io.File.separator), "JsonUtil.java"));
         supportingFiles.add(new SupportingFile("apiException.mustache",
                 (sourceFolder + File.separator + invokerPackage).replace(".", java.io.File.separator), "ApiException.java"));
+        supportingFiles.add(new SupportingFile("Pair.mustache",
+                (sourceFolder + File.separator + invokerPackage).replace(".", java.io.File.separator), "Pair.java"));
     }
 
     public Boolean getUseAndroidMavenGradlePlugin() {
diff --git a/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/JavaClientCodegen.java b/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/JavaClientCodegen.java
index effcddc6834..06b1d90d786 100644
--- a/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/JavaClientCodegen.java
+++ b/modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/JavaClientCodegen.java
@@ -116,6 +116,7 @@ public class JavaClientCodegen extends DefaultCodegen implements CodegenConfig {
         supportingFiles.add(new SupportingFile("Configuration.mustache", invokerFolder, "Configuration.java"));
         supportingFiles.add(new SupportingFile("JsonUtil.mustache", invokerFolder, "JsonUtil.java"));
         supportingFiles.add(new SupportingFile("StringUtil.mustache", invokerFolder, "StringUtil.java"));
+        supportingFiles.add(new SupportingFile("Pair.mustache", invokerFolder, "Pair.java"));
 
         final String authFolder = (sourceFolder + File.separator + invokerPackage + ".auth").replace(".", File.separator);
         supportingFiles.add(new SupportingFile("auth/Authentication.mustache", authFolder, "Authentication.java"));
diff --git a/modules/swagger-codegen/src/main/resources/Java/ApiClient.mustache b/modules/swagger-codegen/src/main/resources/Java/ApiClient.mustache
index e12c835a90a..fb4351d444d 100644
--- a/modules/swagger-codegen/src/main/resources/Java/ApiClient.mustache
+++ b/modules/swagger-codegen/src/main/resources/Java/ApiClient.mustache
@@ -22,6 +22,7 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.HashMap;
 import java.util.List;
+import java.util.ArrayList;
 import java.util.Date;
 import java.util.TimeZone;
 
@@ -238,6 +239,62 @@ public class ApiClient {
     }
   }
 
+  /*
+    Format to {@code Pair} objects.
+  */
+  public List<Pair> parameterToPairs(String collectionFormat, String name, Object value){
+    List<Pair> params = new ArrayList<Pair>();
+
+    // preconditions
+    if (name == null || name.isEmpty() || value == null) return params;
+
+    Collection valueCollection = null;
+    if (value instanceof Collection) {
+      valueCollection = (Collection) value;
+    } else {
+      params.add(new Pair(name, parameterToString(value)));
+      return params;
+    }
+
+    if (valueCollection.isEmpty()){
+      return params;
+    }
+
+    // get the collection format
+    collectionFormat = (collectionFormat == null || collectionFormat.isEmpty() ? "csv" : collectionFormat); // default: csv
+
+    // create the params based on the collection format
+    if (collectionFormat.equals("multi")) {
+      for (Object item : valueCollection) {
+        params.add(new Pair(name, parameterToString(item)));
+      }
+
+      return params;
+    }
+
+    String delimiter = ",";
+
+    if (collectionFormat.equals("csv")) {
+      delimiter = ",";
+    } else if (collectionFormat.equals("ssv")) {
+      delimiter = " ";
+    } else if (collectionFormat.equals("tsv")) {
+      delimiter = "\t";
+    } else if (collectionFormat.equals("pipes")) {
+      delimiter = "|";
+    }
+
+    StringBuilder sb = new StringBuilder() ;
+    for (Object item : valueCollection) {
+      sb.append(delimiter);
+      sb.append(parameterToString(item));
+    }
+
+    params.add(new Pair(name, sb.substring(1)));
+
+    return params;
+  }
+
   /**
    * Select the Accept header's value from the given accepts array:
    *   if JSON exists in the given array, use it;
@@ -341,23 +398,25 @@ public class ApiClient {
    * @param authNames The authentications to apply
    * @return The response body in type of string
    */
-  public String invokeAPI(String path, String method, Map<String, String> queryParams, Object body, Map<String, String> headerParams, Map<String, String> formParams, String accept, String contentType, String[] authNames) throws ApiException {
+  public String invokeAPI(String path, String method, List<Pair> queryParams, Object body, Map<String, String> headerParams, Map<String, String> formParams, String accept, String contentType, String[] authNames) throws ApiException {
     updateParamsForAuth(authNames, queryParams, headerParams);
 
     Client client = getClient();
 
     StringBuilder b = new StringBuilder();
-    for(String key : queryParams.keySet()) {
-      String value = queryParams.get(key);
-      if (value != null){
-        if(b.toString().length() == 0)
-          b.append("?");
-        else
+    b.append("?");
+    if (queryParams != null){
+      for (Pair queryParam : queryParams){
+        if (!queryParam.getName().isEmpty()) {
+          b.append(escapeString(queryParam.getName()));
+          b.append("=");
+          b.append(escapeString(queryParam.getValue()));
           b.append("&");
-        b.append(escapeString(key)).append("=").append(escapeString(value));
+        }
       }
     }
-    String querystring = b.toString();
+
+    String querystring = b.substring(0, b.length() - 1);
 
     Builder builder;
     if (accept == null)
@@ -457,7 +516,7 @@ public class ApiClient {
    *
    * @param authNames The authentications to apply
    */
-  private void updateParamsForAuth(String[] authNames, Map<String, String> queryParams, Map<String, String> headerParams) {
+  private void updateParamsForAuth(String[] authNames, List<Pair> queryParams, Map<String, String> headerParams) {
     for (String authName : authNames) {
       Authentication auth = authentications.get(authName);
       if (auth == null) throw new RuntimeException("Authentication undefined: " + authName);
diff --git a/modules/swagger-codegen/src/main/resources/Java/Pair.mustache b/modules/swagger-codegen/src/main/resources/Java/Pair.mustache
new file mode 100644
index 00000000000..9805c74903b
--- /dev/null
+++ b/modules/swagger-codegen/src/main/resources/Java/Pair.mustache
@@ -0,0 +1,38 @@
+package {{invokerPackage}};
+
+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;
+        if (arg.trim().isEmpty()) return false;
+
+        return true;
+    }
+}
diff --git a/modules/swagger-codegen/src/main/resources/Java/api.mustache b/modules/swagger-codegen/src/main/resources/Java/api.mustache
index 63357afd896..afb28293cad 100644
--- a/modules/swagger-codegen/src/main/resources/Java/api.mustache
+++ b/modules/swagger-codegen/src/main/resources/Java/api.mustache
@@ -3,6 +3,7 @@ package {{package}};
 import {{invokerPackage}}.ApiException;
 import {{invokerPackage}}.ApiClient;
 import {{invokerPackage}}.Configuration;
+import {{invokerPackage}}.Pair;
 
 import {{modelPackage}}.*;
 
@@ -61,16 +62,16 @@ public class {{classname}} {
       .replaceAll("\\{" + "{{paramName}}" + "\\}", apiClient.escapeString({{{paramName}}}.toString())){{/pathParams}};
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
-    {{#queryParams}}if ({{paramName}} != null)
-      queryParams.put("{{baseName}}", apiClient.parameterToString({{paramName}}));
+    {{#queryParams}}
+    queryParams.addAll(apiClient.parameterToPairs("{{#collectionFormat}}{{{collectionFormat}}}{{/collectionFormat}}", "{{baseName}}", {{paramName}}));
     {{/queryParams}}
 
     {{#headerParams}}if ({{paramName}} != null)
-      headerParams.put("{{baseName}}", apiClient.parameterToString({{paramName}}));
+    headerParams.put("{{baseName}}", apiClient.parameterToString({{paramName}}));
     {{/headerParams}}
 
     final String[] accepts = {
diff --git a/modules/swagger-codegen/src/main/resources/Java/auth/ApiKeyAuth.mustache b/modules/swagger-codegen/src/main/resources/Java/auth/ApiKeyAuth.mustache
index 65720b958cb..a1824b551ca 100644
--- a/modules/swagger-codegen/src/main/resources/Java/auth/ApiKeyAuth.mustache
+++ b/modules/swagger-codegen/src/main/resources/Java/auth/ApiKeyAuth.mustache
@@ -1,6 +1,9 @@
 package {{invokerPackage}}.auth;
 
+import {{invokerPackage}}.Pair;
+
 import java.util.Map;
+import java.util.List;
 
 public class ApiKeyAuth implements Authentication {
   private final String location;
@@ -39,7 +42,7 @@ public class ApiKeyAuth implements Authentication {
   }
 
   @Override
-  public void applyToParams(Map<String, String> queryParams, Map<String, String> headerParams) {
+  public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams) {
     String value;
     if (apiKeyPrefix != null) {
       value = apiKeyPrefix + " " + apiKey;
@@ -47,7 +50,7 @@ public class ApiKeyAuth implements Authentication {
       value = apiKey;
     }
     if (location == "query") {
-      queryParams.put(paramName, value);
+      queryParams.add(new Pair(paramName, value));
     } else if (location == "header") {
       headerParams.put(paramName, value);
     }
diff --git a/modules/swagger-codegen/src/main/resources/Java/auth/Authentication.mustache b/modules/swagger-codegen/src/main/resources/Java/auth/Authentication.mustache
index 1b2e2bc2fbe..265c74cb76f 100644
--- a/modules/swagger-codegen/src/main/resources/Java/auth/Authentication.mustache
+++ b/modules/swagger-codegen/src/main/resources/Java/auth/Authentication.mustache
@@ -1,8 +1,11 @@
 package {{invokerPackage}}.auth;
 
+import {{invokerPackage}}.Pair;
+
 import java.util.Map;
+import java.util.List;
 
 public interface Authentication {
   /** Apply authentication settings to header and query params. */
-  void applyToParams(Map<String, String> queryParams, Map<String, String> headerParams);
+  void applyToParams(List<Pair> queryParams, Map<String, String> headerParams);
 }
diff --git a/modules/swagger-codegen/src/main/resources/Java/auth/HttpBasicAuth.mustache b/modules/swagger-codegen/src/main/resources/Java/auth/HttpBasicAuth.mustache
index 22a64b1a24e..032ea57d4e8 100644
--- a/modules/swagger-codegen/src/main/resources/Java/auth/HttpBasicAuth.mustache
+++ b/modules/swagger-codegen/src/main/resources/Java/auth/HttpBasicAuth.mustache
@@ -1,6 +1,9 @@
 package {{invokerPackage}}.auth;
 
+import {{invokerPackage}}.Pair;
+
 import java.util.Map;
+import java.util.List;
 
 import java.io.UnsupportedEncodingException;
 import javax.xml.bind.DatatypeConverter;
@@ -26,7 +29,7 @@ public class HttpBasicAuth implements Authentication {
   }
 
   @Override
-  public void applyToParams(Map<String, String> queryParams, Map<String, String> headerParams) {
+  public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams) {
     String str = (username == null ? "" : username) + ":" + (password == null ? "" : password);
     try {
       headerParams.put("Authorization", "Basic " + DatatypeConverter.printBase64Binary(str.getBytes("UTF-8")));
diff --git a/modules/swagger-codegen/src/main/resources/Java/auth/OAuth.mustache b/modules/swagger-codegen/src/main/resources/Java/auth/OAuth.mustache
index ef84b8cc05e..66cf2ac8f0f 100644
--- a/modules/swagger-codegen/src/main/resources/Java/auth/OAuth.mustache
+++ b/modules/swagger-codegen/src/main/resources/Java/auth/OAuth.mustache
@@ -1,10 +1,13 @@
 package {{invokerPackage}}.auth;
 
+import {{invokerPackage}}.Pair;
+
 import java.util.Map;
+import java.util.List;
 
 public class OAuth implements Authentication {
   @Override
-  public void applyToParams(Map<String, String> queryParams, Map<String, String> headerParams) {
+  public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams) {
     // TODO: support oauth
   }
 }
diff --git a/modules/swagger-codegen/src/main/resources/android-java/Pair.mustache b/modules/swagger-codegen/src/main/resources/android-java/Pair.mustache
new file mode 100644
index 00000000000..5456028a1a0
--- /dev/null
+++ b/modules/swagger-codegen/src/main/resources/android-java/Pair.mustache
@@ -0,0 +1,38 @@
+package {{invokerPackage}};
+
+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;
+        if (arg.trim().isEmpty()) return false;
+
+        return true;
+    }
+}
diff --git a/modules/swagger-codegen/src/main/resources/android-java/api.mustache b/modules/swagger-codegen/src/main/resources/android-java/api.mustache
index ed9538c1594..9f341301cf0 100644
--- a/modules/swagger-codegen/src/main/resources/android-java/api.mustache
+++ b/modules/swagger-codegen/src/main/resources/android-java/api.mustache
@@ -2,6 +2,7 @@ package {{package}};
 
 import {{invokerPackage}}.ApiException;
 import {{invokerPackage}}.ApiInvoker;
+import {{invokerPackage}}.Pair;
 
 import {{modelPackage}}.*;
 
@@ -58,17 +59,18 @@ public class {{classname}} {
     String path = "{{path}}".replaceAll("\\{format\\}","json"){{#pathParams}}.replaceAll("\\{" + "{{paramName}}" + "\\}", apiInvoker.escapeString({{{paramName}}}.toString())){{/pathParams}};
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
     Map<String, String> formParams = new HashMap<String, String>();
 
-    {{#queryParams}}if ({{paramName}} != null)
-      queryParams.put("{{baseName}}", ApiInvoker.parameterToString({{paramName}}));
+    {{#queryParams}}
+    queryParams.addAll(ApiInvoker.parameterToPairs("{{#collectionFormat}}{{{collectionFormat}}}{{/collectionFormat}}", "{{baseName}}", {{paramName}}));
     {{/queryParams}}
 
-    {{#headerParams}}headerParams.put("{{baseName}}", ApiInvoker.parameterToString({{paramName}}));
+    {{#headerParams}}
+    headerParams.put("{{baseName}}", ApiInvoker.parameterToString({{paramName}}));
     {{/headerParams}}
 
     String[] contentTypes = {
diff --git a/modules/swagger-codegen/src/main/resources/android-java/apiInvoker.mustache b/modules/swagger-codegen/src/main/resources/android-java/apiInvoker.mustache
index e759f8264a4..5aabd2458e8 100644
--- a/modules/swagger-codegen/src/main/resources/android-java/apiInvoker.mustache
+++ b/modules/swagger-codegen/src/main/resources/android-java/apiInvoker.mustache
@@ -23,6 +23,7 @@ import java.util.Collection;
 import java.util.Map;
 import java.util.HashMap;
 import java.util.List;
+import java.util.ArrayList;
 import java.io.IOException;
 import java.io.UnsupportedEncodingException;
 
@@ -132,6 +133,62 @@ public class ApiInvoker {
     }
   }
 
+  /*
+    Format to {@code Pair} objects.
+  */
+  public static List<Pair> parameterToPairs(String collectionFormat, String name, Object value){
+    List<Pair> params = new ArrayList<Pair>();
+
+    // preconditions
+    if (name == null || name.isEmpty() || value == null) return params;
+
+    Collection valueCollection = null;
+    if (value instanceof Collection) {
+      valueCollection = (Collection) value;
+    } else {
+      params.add(new Pair(name, parameterToString(value)));
+      return params;
+    }
+
+    if (valueCollection.isEmpty()){
+      return params;
+    }
+
+    // get the collection format
+    collectionFormat = (collectionFormat == null || collectionFormat.isEmpty() ? "csv" : collectionFormat); // default: csv
+
+    // create the params based on the collection format
+    if (collectionFormat.equals("multi")) {
+      for (Object item : valueCollection) {
+        params.add(new Pair(name, parameterToString(item)));
+      }
+
+      return params;
+    }
+
+    String delimiter = ",";
+
+    if (collectionFormat.equals("csv")) {
+      delimiter = ",";
+    } else if (collectionFormat.equals("ssv")) {
+      delimiter = " ";
+    } else if (collectionFormat.equals("tsv")) {
+      delimiter = "\t";
+    } else if (collectionFormat.equals("pipes")) {
+      delimiter = "|";
+    }
+
+    StringBuilder sb = new StringBuilder() ;
+    for (Object item : valueCollection) {
+      sb.append(delimiter);
+      sb.append(parameterToString(item));
+    }
+
+    params.add(new Pair(name, sb.substring(1)));
+
+    return params;
+  }
+
   public ApiInvoker() {
     initConnectionManager();
   }
@@ -184,21 +241,24 @@ public class ApiInvoker {
     }
   }
 
-  public String invokeAPI(String host, String path, String method, Map<String, String> queryParams, Object body, Map<String, String> headerParams, Map<String, String> formParams, String contentType) throws ApiException {
+  public String invokeAPI(String host, String path, String method, List<Pair> queryParams, Object body, Map<String, String> headerParams, Map<String, String> formParams, String contentType) throws ApiException {
     HttpClient client = getClient(host);
 
     StringBuilder b = new StringBuilder();
-    for(String key : queryParams.keySet()) {
-      String value = queryParams.get(key);
-      if (value != null){
-        if(b.toString().length() == 0)
-          b.append("?");
-        else
+    b.append("?");
+    if (queryParams != null){
+      for (Pair queryParam : queryParams){
+        if (!queryParam.getName().isEmpty()) {
+          b.append(escapeString(queryParam.getName()));
+          b.append("=");
+          b.append(escapeString(queryParam.getValue()));
           b.append("&");
-        b.append(escapeString(key)).append("=").append(escapeString(value));
+        }
       }
     }
-    String url = host + path + b.toString();
+
+    String querystring = b.substring(0, b.length() - 1);
+    String url = host + path + querystring;
 
     HashMap<String, String> headers = new HashMap<String, String>();
 
diff --git a/samples/client/petstore/android-java/src/main/java/io/swagger/client/ApiInvoker.java b/samples/client/petstore/android-java/src/main/java/io/swagger/client/ApiInvoker.java
index ec6218e602c..0ccc3713958 100644
--- a/samples/client/petstore/android-java/src/main/java/io/swagger/client/ApiInvoker.java
+++ b/samples/client/petstore/android-java/src/main/java/io/swagger/client/ApiInvoker.java
@@ -23,6 +23,7 @@ import java.util.Collection;
 import java.util.Map;
 import java.util.HashMap;
 import java.util.List;
+import java.util.ArrayList;
 import java.io.IOException;
 import java.io.UnsupportedEncodingException;
 
@@ -132,6 +133,62 @@ public class ApiInvoker {
     }
   }
 
+  /*
+    Format to {@code Pair} objects.
+  */
+  public static List<Pair> parameterToPairs(String collectionFormat, String name, Object value){
+    List<Pair> params = new ArrayList<Pair>();
+
+    // preconditions
+    if (name == null || name.isEmpty() || value == null) return params;
+
+    Collection valueCollection = null;
+    if (value instanceof Collection) {
+      valueCollection = (Collection) value;
+    } else {
+      params.add(new Pair(name, parameterToString(value)));
+      return params;
+    }
+
+    if (valueCollection.isEmpty()){
+      return params;
+    }
+
+    // get the collection format
+    collectionFormat = (collectionFormat == null || collectionFormat.isEmpty() ? "csv" : collectionFormat); // default: csv
+
+    // create the params based on the collection format
+    if (collectionFormat.equals("multi")) {
+      for (Object item : valueCollection) {
+        params.add(new Pair(name, parameterToString(item)));
+      }
+
+      return params;
+    }
+
+    String delimiter = ",";
+
+    if (collectionFormat.equals("csv")) {
+      delimiter = ",";
+    } else if (collectionFormat.equals("ssv")) {
+      delimiter = " ";
+    } else if (collectionFormat.equals("tsv")) {
+      delimiter = "\t";
+    } else if (collectionFormat.equals("pipes")) {
+      delimiter = "|";
+    }
+
+    StringBuilder sb = new StringBuilder() ;
+    for (Object item : valueCollection) {
+      sb.append(delimiter);
+      sb.append(parameterToString(item));
+    }
+
+    params.add(new Pair(name, sb.substring(1)));
+
+    return params;
+  }
+
   public ApiInvoker() {
     initConnectionManager();
   }
@@ -184,21 +241,24 @@ public class ApiInvoker {
     }
   }
 
-  public String invokeAPI(String host, String path, String method, Map<String, String> queryParams, Object body, Map<String, String> headerParams, Map<String, String> formParams, String contentType) throws ApiException {
+  public String invokeAPI(String host, String path, String method, List<Pair> queryParams, Object body, Map<String, String> headerParams, Map<String, String> formParams, String contentType) throws ApiException {
     HttpClient client = getClient(host);
 
     StringBuilder b = new StringBuilder();
-    for(String key : queryParams.keySet()) {
-      String value = queryParams.get(key);
-      if (value != null){
-        if(b.toString().length() == 0)
-          b.append("?");
-        else
+    b.append("?");
+    if (queryParams != null){
+      for (Pair queryParam : queryParams){
+        if (!queryParam.getName().isEmpty()) {
+          b.append(escapeString(queryParam.getName()));
+          b.append("=");
+          b.append(escapeString(queryParam.getValue()));
           b.append("&");
-        b.append(escapeString(key)).append("=").append(escapeString(value));
+        }
       }
     }
-    String url = host + path + b.toString();
+
+    String querystring = b.substring(0, b.length() - 1);
+    String url = host + path + querystring;
 
     HashMap<String, String> headers = new HashMap<String, String>();
 
diff --git a/samples/client/petstore/android-java/src/main/java/io/swagger/client/Pair.java b/samples/client/petstore/android-java/src/main/java/io/swagger/client/Pair.java
new file mode 100644
index 00000000000..2710fb5a99b
--- /dev/null
+++ b/samples/client/petstore/android-java/src/main/java/io/swagger/client/Pair.java
@@ -0,0 +1,38 @@
+package io.swagger.client;
+
+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;
+        if (arg.trim().isEmpty()) return false;
+
+        return true;
+    }
+}
diff --git a/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/PetApi.java b/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/PetApi.java
index ebf600d787e..21ee69d089b 100644
--- a/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/PetApi.java
+++ b/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/PetApi.java
@@ -2,6 +2,7 @@ package io.swagger.client.api;
 
 import io.swagger.client.ApiException;
 import io.swagger.client.ApiInvoker;
+import io.swagger.client.Pair;
 
 import io.swagger.client.model.*;
 
@@ -52,7 +53,7 @@ public class PetApi {
     String path = "/pet".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -106,7 +107,7 @@ public class PetApi {
     String path = "/pet".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -160,14 +161,14 @@ public class PetApi {
     String path = "/pet/findByStatus".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
     Map<String, String> formParams = new HashMap<String, String>();
 
-    if (status != null)
-      queryParams.put("status", ApiInvoker.parameterToString(status));
+    
+    queryParams.addAll(ApiInvoker.parameterToPairs("multi", "status", status));
     
 
     
@@ -216,14 +217,14 @@ public class PetApi {
     String path = "/pet/findByTags".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
     Map<String, String> formParams = new HashMap<String, String>();
 
-    if (tags != null)
-      queryParams.put("tags", ApiInvoker.parameterToString(tags));
+    
+    queryParams.addAll(ApiInvoker.parameterToPairs("multi", "tags", tags));
     
 
     
@@ -277,7 +278,7 @@ public class PetApi {
     String path = "/pet/{petId}".replaceAll("\\{format\\}","json").replaceAll("\\{" + "petId" + "\\}", apiInvoker.escapeString(petId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -338,7 +339,7 @@ public class PetApi {
     String path = "/pet/{petId}".replaceAll("\\{format\\}","json").replaceAll("\\{" + "petId" + "\\}", apiInvoker.escapeString(petId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -408,7 +409,7 @@ public class PetApi {
     String path = "/pet/{petId}".replaceAll("\\{format\\}","json").replaceAll("\\{" + "petId" + "\\}", apiInvoker.escapeString(petId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -416,6 +417,7 @@ public class PetApi {
 
     
 
+    
     headerParams.put("api_key", ApiInvoker.parameterToString(apiKey));
     
 
@@ -470,7 +472,7 @@ public class PetApi {
     String path = "/pet/{petId}/uploadImage".replaceAll("\\{format\\}","json").replaceAll("\\{" + "petId" + "\\}", apiInvoker.escapeString(petId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
diff --git a/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/StoreApi.java b/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/StoreApi.java
index ecd8e833049..49a3ac4aa9a 100644
--- a/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/StoreApi.java
+++ b/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/StoreApi.java
@@ -2,6 +2,7 @@ package io.swagger.client.api;
 
 import io.swagger.client.ApiException;
 import io.swagger.client.ApiInvoker;
+import io.swagger.client.Pair;
 
 import io.swagger.client.model.*;
 
@@ -51,7 +52,7 @@ public class StoreApi {
     String path = "/store/inventory".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -105,7 +106,7 @@ public class StoreApi {
     String path = "/store/order".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -164,7 +165,7 @@ public class StoreApi {
     String path = "/store/order/{orderId}".replaceAll("\\{format\\}","json").replaceAll("\\{" + "orderId" + "\\}", apiInvoker.escapeString(orderId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -223,7 +224,7 @@ public class StoreApi {
     String path = "/store/order/{orderId}".replaceAll("\\{format\\}","json").replaceAll("\\{" + "orderId" + "\\}", apiInvoker.escapeString(orderId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
diff --git a/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/UserApi.java b/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/UserApi.java
index e1df3fd4f12..a16a745c7f8 100644
--- a/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/UserApi.java
+++ b/samples/client/petstore/android-java/src/main/java/io/swagger/client/api/UserApi.java
@@ -2,6 +2,7 @@ package io.swagger.client.api;
 
 import io.swagger.client.ApiException;
 import io.swagger.client.ApiInvoker;
+import io.swagger.client.Pair;
 
 import io.swagger.client.model.*;
 
@@ -52,7 +53,7 @@ public class UserApi {
     String path = "/user".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -106,7 +107,7 @@ public class UserApi {
     String path = "/user/createWithArray".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -160,7 +161,7 @@ public class UserApi {
     String path = "/user/createWithList".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -215,16 +216,16 @@ public class UserApi {
     String path = "/user/login".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
     Map<String, String> formParams = new HashMap<String, String>();
 
-    if (username != null)
-      queryParams.put("username", ApiInvoker.parameterToString(username));
-    if (password != null)
-      queryParams.put("password", ApiInvoker.parameterToString(password));
+    
+    queryParams.addAll(ApiInvoker.parameterToPairs("", "username", username));
+    
+    queryParams.addAll(ApiInvoker.parameterToPairs("", "password", password));
     
 
     
@@ -272,7 +273,7 @@ public class UserApi {
     String path = "/user/logout".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -331,7 +332,7 @@ public class UserApi {
     String path = "/user/{username}".replaceAll("\\{format\\}","json").replaceAll("\\{" + "username" + "\\}", apiInvoker.escapeString(username.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -391,7 +392,7 @@ public class UserApi {
     String path = "/user/{username}".replaceAll("\\{format\\}","json").replaceAll("\\{" + "username" + "\\}", apiInvoker.escapeString(username.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
@@ -450,7 +451,7 @@ public class UserApi {
     String path = "/user/{username}".replaceAll("\\{format\\}","json").replaceAll("\\{" + "username" + "\\}", apiInvoker.escapeString(username.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     // header params
     Map<String, String> headerParams = new HashMap<String, String>();
     // form params
diff --git a/samples/client/petstore/android-java/src/test/java/io/swagger/client/ApiInvokerTest.java b/samples/client/petstore/android-java/src/test/java/io/swagger/client/ApiInvokerTest.java
new file mode 100644
index 00000000000..3e4b114012c
--- /dev/null
+++ b/samples/client/petstore/android-java/src/test/java/io/swagger/client/ApiInvokerTest.java
@@ -0,0 +1,91 @@
+package io.swagger.client;
+
+import org.junit.*;
+import static org.junit.Assert.*;
+
+import java.util.*;
+
+
+public class ApiInvokerTest {
+
+    @Test
+    public void testParameterToPairsWhenNameIsInvalid() throws Exception {
+        List<Pair> pairs_a = ApiInvoker.parameterToPairs("csv", null, new Integer(1));
+        List<Pair> pairs_b = ApiInvoker.parameterToPairs("csv", "", new Integer(1));
+
+        assertTrue(pairs_a.isEmpty());
+        assertTrue(pairs_b.isEmpty());
+    }
+
+    @Test
+    public void testParameterToPairsWhenValueIsNull() throws Exception {
+        List<Pair> pairs = ApiInvoker.parameterToPairs("csv", "param-a", null);
+
+        assertTrue(pairs.isEmpty());
+    }
+
+    @Test
+    public void testParameterToPairsWhenValueIsEmptyStrings() throws Exception {
+
+        // single empty string
+        List<Pair> pairs = ApiInvoker.parameterToPairs("csv", "param-a", " ");
+        assertEquals(1, pairs.size());
+
+        // list of empty strings
+        List<String> strs = new ArrayList<String>();
+        strs.add(" ");
+        strs.add(" ");
+        strs.add(" ");
+
+        List<Pair> concatStrings = ApiInvoker.parameterToPairs("csv", "param-a", strs);
+
+        assertEquals(1, concatStrings.size());
+        assertFalse(concatStrings.get(0).getValue().isEmpty()); // should contain some delimiters
+    }
+
+    @Test
+    public void testParameterToPairsWhenValueIsNotCollection() throws Exception {
+        String name = "param-a";
+        Integer value = 1;
+
+        List<Pair> pairs = ApiInvoker.parameterToPairs("csv", name, value);
+
+        assertEquals(1, pairs.size());
+        assertEquals(value, Integer.valueOf(pairs.get(0).getValue()));
+    }
+
+    @Test
+    public void testParameterToPairsWhenValueIsCollection() throws Exception {
+        Map<String, String> collectionFormatMap = new HashMap<String, String>();
+        collectionFormatMap.put("csv", ",");
+        collectionFormatMap.put("tsv", "\t");
+        collectionFormatMap.put("ssv", " ");
+        collectionFormatMap.put("pipes", "\\|");
+        collectionFormatMap.put("", ","); // no format, must default to csv
+        collectionFormatMap.put("unknown", ","); // all other formats, must default to csv
+
+        String name = "param-a";
+
+        List<Object> values = new ArrayList<Object>();
+        values.add("value-a");
+        values.add(123);
+        values.add(new Date());
+
+        // check for multi separately
+        List<Pair> multiPairs = ApiInvoker.parameterToPairs("multi", name, values);
+        assertEquals(values.size(), multiPairs.size());
+
+        // all other formats
+        for (String collectionFormat : collectionFormatMap.keySet()) {
+            List<Pair> pairs = ApiInvoker.parameterToPairs(collectionFormat, name, values);
+
+            assertEquals(1, pairs.size());
+
+            String delimiter = collectionFormatMap.get(collectionFormat);
+            String[] pairValueSplit = pairs.get(0).getValue().split(delimiter);
+
+            // must equal input values
+            assertEquals(values.size(), pairValueSplit.length);
+        }
+    }
+}
diff --git a/samples/client/petstore/java/src/main/java/io/swagger/client/ApiClient.java b/samples/client/petstore/java/src/main/java/io/swagger/client/ApiClient.java
index a95f5f7a960..87ecc831251 100644
--- a/samples/client/petstore/java/src/main/java/io/swagger/client/ApiClient.java
+++ b/samples/client/petstore/java/src/main/java/io/swagger/client/ApiClient.java
@@ -22,6 +22,7 @@ import java.util.Map;
 import java.util.Map.Entry;
 import java.util.HashMap;
 import java.util.List;
+import java.util.ArrayList;
 import java.util.Date;
 import java.util.TimeZone;
 
@@ -237,6 +238,62 @@ public class ApiClient {
     }
   }
 
+  /*
+    Format to {@code Pair} objects.
+  */
+  public List<Pair> parameterToPairs(String collectionFormat, String name, Object value){
+    List<Pair> params = new ArrayList<Pair>();
+
+    // preconditions
+    if (name == null || name.isEmpty() || value == null) return params;
+
+    Collection valueCollection = null;
+    if (value instanceof Collection) {
+      valueCollection = (Collection) value;
+    } else {
+      params.add(new Pair(name, parameterToString(value)));
+      return params;
+    }
+
+    if (valueCollection.isEmpty()){
+      return params;
+    }
+
+    // get the collection format
+    collectionFormat = (collectionFormat == null || collectionFormat.isEmpty() ? "csv" : collectionFormat); // default: csv
+
+    // create the params based on the collection format
+    if (collectionFormat.equals("multi")) {
+      for (Object item : valueCollection) {
+        params.add(new Pair(name, parameterToString(item)));
+      }
+
+      return params;
+    }
+
+    String delimiter = ",";
+
+    if (collectionFormat.equals("csv")) {
+      delimiter = ",";
+    } else if (collectionFormat.equals("ssv")) {
+      delimiter = " ";
+    } else if (collectionFormat.equals("tsv")) {
+      delimiter = "\t";
+    } else if (collectionFormat.equals("pipes")) {
+      delimiter = "|";
+    }
+
+    StringBuilder sb = new StringBuilder() ;
+    for (Object item : valueCollection) {
+      sb.append(delimiter);
+      sb.append(parameterToString(item));
+    }
+
+    params.add(new Pair(name, sb.substring(1)));
+
+    return params;
+  }
+
   /**
    * Select the Accept header's value from the given accepts array:
    *   if JSON exists in the given array, use it;
@@ -340,23 +397,25 @@ public class ApiClient {
    * @param authNames The authentications to apply
    * @return The response body in type of string
    */
-  public String invokeAPI(String path, String method, Map<String, String> queryParams, Object body, Map<String, String> headerParams, Map<String, String> formParams, String accept, String contentType, String[] authNames) throws ApiException {
+  public String invokeAPI(String path, String method, List<Pair> queryParams, Object body, Map<String, String> headerParams, Map<String, String> formParams, String accept, String contentType, String[] authNames) throws ApiException {
     updateParamsForAuth(authNames, queryParams, headerParams);
 
     Client client = getClient();
 
     StringBuilder b = new StringBuilder();
-    for(String key : queryParams.keySet()) {
-      String value = queryParams.get(key);
-      if (value != null){
-        if(b.toString().length() == 0)
-          b.append("?");
-        else
+    b.append("?");
+    if (queryParams != null){
+      for (Pair queryParam : queryParams){
+        if (!queryParam.getName().isEmpty()) {
+          b.append(escapeString(queryParam.getName()));
+          b.append("=");
+          b.append(escapeString(queryParam.getValue()));
           b.append("&");
-        b.append(escapeString(key)).append("=").append(escapeString(value));
+        }
       }
     }
-    String querystring = b.toString();
+
+    String querystring = b.substring(0, b.length() - 1);
 
     Builder builder;
     if (accept == null)
@@ -456,7 +515,7 @@ public class ApiClient {
    *
    * @param authNames The authentications to apply
    */
-  private void updateParamsForAuth(String[] authNames, Map<String, String> queryParams, Map<String, String> headerParams) {
+  private void updateParamsForAuth(String[] authNames, List<Pair> queryParams, Map<String, String> headerParams) {
     for (String authName : authNames) {
       Authentication auth = authentications.get(authName);
       if (auth == null) throw new RuntimeException("Authentication undefined: " + authName);
diff --git a/samples/client/petstore/java/src/main/java/io/swagger/client/Pair.java b/samples/client/petstore/java/src/main/java/io/swagger/client/Pair.java
new file mode 100644
index 00000000000..4b7112f6db6
--- /dev/null
+++ b/samples/client/petstore/java/src/main/java/io/swagger/client/Pair.java
@@ -0,0 +1,38 @@
+package io.swagger.client;
+
+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;
+        if (arg.trim().isEmpty()) return false;
+
+        return true;
+    }
+}
diff --git a/samples/client/petstore/java/src/main/java/io/swagger/client/api/PetApi.java b/samples/client/petstore/java/src/main/java/io/swagger/client/api/PetApi.java
index bc793fd41ee..402fc9da953 100644
--- a/samples/client/petstore/java/src/main/java/io/swagger/client/api/PetApi.java
+++ b/samples/client/petstore/java/src/main/java/io/swagger/client/api/PetApi.java
@@ -3,6 +3,7 @@ package io.swagger.client.api;
 import io.swagger.client.ApiException;
 import io.swagger.client.ApiClient;
 import io.swagger.client.Configuration;
+import io.swagger.client.Pair;
 
 import io.swagger.client.model.*;
 
@@ -54,7 +55,7 @@ public class PetApi {
     String path = "/pet".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -111,7 +112,7 @@ public class PetApi {
     String path = "/pet".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -168,12 +169,12 @@ public class PetApi {
     String path = "/pet/findByStatus".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
-    if (status != null)
-      queryParams.put("status", apiClient.parameterToString(status));
+    
+    queryParams.addAll(apiClient.parameterToPairs("multi", "status", status));
     
 
     
@@ -227,12 +228,12 @@ public class PetApi {
     String path = "/pet/findByTags".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
-    if (tags != null)
-      queryParams.put("tags", apiClient.parameterToString(tags));
+    
+    queryParams.addAll(apiClient.parameterToPairs("multi", "tags", tags));
     
 
     
@@ -292,7 +293,7 @@ public class PetApi {
       .replaceAll("\\{" + "petId" + "\\}", apiClient.escapeString(petId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -357,7 +358,7 @@ public class PetApi {
       .replaceAll("\\{" + "petId" + "\\}", apiClient.escapeString(petId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -435,14 +436,14 @@ public class PetApi {
       .replaceAll("\\{" + "petId" + "\\}", apiClient.escapeString(petId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
     
 
     if (apiKey != null)
-      headerParams.put("api_key", apiClient.parameterToString(apiKey));
+    headerParams.put("api_key", apiClient.parameterToString(apiKey));
     
 
     final String[] accepts = {
@@ -502,7 +503,7 @@ public class PetApi {
       .replaceAll("\\{" + "petId" + "\\}", apiClient.escapeString(petId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
diff --git a/samples/client/petstore/java/src/main/java/io/swagger/client/api/StoreApi.java b/samples/client/petstore/java/src/main/java/io/swagger/client/api/StoreApi.java
index 727e791c603..498b6d4a6b8 100644
--- a/samples/client/petstore/java/src/main/java/io/swagger/client/api/StoreApi.java
+++ b/samples/client/petstore/java/src/main/java/io/swagger/client/api/StoreApi.java
@@ -3,6 +3,7 @@ package io.swagger.client.api;
 import io.swagger.client.ApiException;
 import io.swagger.client.ApiClient;
 import io.swagger.client.Configuration;
+import io.swagger.client.Pair;
 
 import io.swagger.client.model.*;
 
@@ -53,7 +54,7 @@ public class StoreApi {
     String path = "/store/inventory".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -110,7 +111,7 @@ public class StoreApi {
     String path = "/store/order".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -173,7 +174,7 @@ public class StoreApi {
       .replaceAll("\\{" + "orderId" + "\\}", apiClient.escapeString(orderId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -236,7 +237,7 @@ public class StoreApi {
       .replaceAll("\\{" + "orderId" + "\\}", apiClient.escapeString(orderId.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
diff --git a/samples/client/petstore/java/src/main/java/io/swagger/client/api/UserApi.java b/samples/client/petstore/java/src/main/java/io/swagger/client/api/UserApi.java
index 9e55bbed590..3bff204ac02 100644
--- a/samples/client/petstore/java/src/main/java/io/swagger/client/api/UserApi.java
+++ b/samples/client/petstore/java/src/main/java/io/swagger/client/api/UserApi.java
@@ -3,6 +3,7 @@ package io.swagger.client.api;
 import io.swagger.client.ApiException;
 import io.swagger.client.ApiClient;
 import io.swagger.client.Configuration;
+import io.swagger.client.Pair;
 
 import io.swagger.client.model.*;
 
@@ -54,7 +55,7 @@ public class UserApi {
     String path = "/user".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -111,7 +112,7 @@ public class UserApi {
     String path = "/user/createWithArray".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -168,7 +169,7 @@ public class UserApi {
     String path = "/user/createWithList".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -226,14 +227,14 @@ public class UserApi {
     String path = "/user/login".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
-    if (username != null)
-      queryParams.put("username", apiClient.parameterToString(username));
-    if (password != null)
-      queryParams.put("password", apiClient.parameterToString(password));
+    
+    queryParams.addAll(apiClient.parameterToPairs("", "username", username));
+    
+    queryParams.addAll(apiClient.parameterToPairs("", "password", password));
     
 
     
@@ -286,7 +287,7 @@ public class UserApi {
     String path = "/user/logout".replaceAll("\\{format\\}","json");
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -349,7 +350,7 @@ public class UserApi {
       .replaceAll("\\{" + "username" + "\\}", apiClient.escapeString(username.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -413,7 +414,7 @@ public class UserApi {
       .replaceAll("\\{" + "username" + "\\}", apiClient.escapeString(username.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
@@ -476,7 +477,7 @@ public class UserApi {
       .replaceAll("\\{" + "username" + "\\}", apiClient.escapeString(username.toString()));
 
     // query params
-    Map<String, String> queryParams = new HashMap<String, String>();
+    List<Pair> queryParams = new ArrayList<Pair>();
     Map<String, String> headerParams = new HashMap<String, String>();
     Map<String, String> formParams = new HashMap<String, String>();
 
diff --git a/samples/client/petstore/java/src/main/java/io/swagger/client/auth/ApiKeyAuth.java b/samples/client/petstore/java/src/main/java/io/swagger/client/auth/ApiKeyAuth.java
index ce55babb51d..0e5ca9c7c53 100644
--- a/samples/client/petstore/java/src/main/java/io/swagger/client/auth/ApiKeyAuth.java
+++ b/samples/client/petstore/java/src/main/java/io/swagger/client/auth/ApiKeyAuth.java
@@ -1,6 +1,9 @@
 package io.swagger.client.auth;
 
+import io.swagger.client.Pair;
+
 import java.util.Map;
+import java.util.List;
 
 public class ApiKeyAuth implements Authentication {
   private final String location;
@@ -39,7 +42,7 @@ public class ApiKeyAuth implements Authentication {
   }
 
   @Override
-  public void applyToParams(Map<String, String> queryParams, Map<String, String> headerParams) {
+  public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams) {
     String value;
     if (apiKeyPrefix != null) {
       value = apiKeyPrefix + " " + apiKey;
@@ -47,7 +50,7 @@ public class ApiKeyAuth implements Authentication {
       value = apiKey;
     }
     if (location == "query") {
-      queryParams.put(paramName, value);
+      queryParams.add(new Pair(paramName, value));
     } else if (location == "header") {
       headerParams.put(paramName, value);
     }
diff --git a/samples/client/petstore/java/src/main/java/io/swagger/client/auth/Authentication.java b/samples/client/petstore/java/src/main/java/io/swagger/client/auth/Authentication.java
index 3f372404c8d..98b1a6900b9 100644
--- a/samples/client/petstore/java/src/main/java/io/swagger/client/auth/Authentication.java
+++ b/samples/client/petstore/java/src/main/java/io/swagger/client/auth/Authentication.java
@@ -1,8 +1,11 @@
 package io.swagger.client.auth;
 
+import io.swagger.client.Pair;
+
 import java.util.Map;
+import java.util.List;
 
 public interface Authentication {
   /** Apply authentication settings to header and query params. */
-  void applyToParams(Map<String, String> queryParams, Map<String, String> headerParams);
+  void applyToParams(List<Pair> queryParams, Map<String, String> headerParams);
 }
diff --git a/samples/client/petstore/java/src/main/java/io/swagger/client/auth/HttpBasicAuth.java b/samples/client/petstore/java/src/main/java/io/swagger/client/auth/HttpBasicAuth.java
index 24bff8c2266..980b24311be 100644
--- a/samples/client/petstore/java/src/main/java/io/swagger/client/auth/HttpBasicAuth.java
+++ b/samples/client/petstore/java/src/main/java/io/swagger/client/auth/HttpBasicAuth.java
@@ -1,6 +1,9 @@
 package io.swagger.client.auth;
 
+import io.swagger.client.Pair;
+
 import java.util.Map;
+import java.util.List;
 
 import java.io.UnsupportedEncodingException;
 import javax.xml.bind.DatatypeConverter;
@@ -26,7 +29,7 @@ public class HttpBasicAuth implements Authentication {
   }
 
   @Override
-  public void applyToParams(Map<String, String> queryParams, Map<String, String> headerParams) {
+  public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams) {
     String str = (username == null ? "" : username) + ":" + (password == null ? "" : password);
     try {
       headerParams.put("Authorization", "Basic " + DatatypeConverter.printBase64Binary(str.getBytes("UTF-8")));
diff --git a/samples/client/petstore/java/src/main/java/io/swagger/client/auth/OAuth.java b/samples/client/petstore/java/src/main/java/io/swagger/client/auth/OAuth.java
index d834f4580c2..39fba5498c0 100644
--- a/samples/client/petstore/java/src/main/java/io/swagger/client/auth/OAuth.java
+++ b/samples/client/petstore/java/src/main/java/io/swagger/client/auth/OAuth.java
@@ -1,10 +1,13 @@
 package io.swagger.client.auth;
 
+import io.swagger.client.Pair;
+
 import java.util.Map;
+import java.util.List;
 
 public class OAuth implements Authentication {
   @Override
-  public void applyToParams(Map<String, String> queryParams, Map<String, String> headerParams) {
+  public void applyToParams(List<Pair> queryParams, Map<String, String> headerParams) {
     // TODO: support oauth
   }
 }
diff --git a/samples/client/petstore/java/src/test/java/io/swagger/client/ApiClientTest.java b/samples/client/petstore/java/src/test/java/io/swagger/client/ApiClientTest.java
index 8a560d2d7bc..802c0cae3b9 100644
--- a/samples/client/petstore/java/src/test/java/io/swagger/client/ApiClientTest.java
+++ b/samples/client/petstore/java/src/test/java/io/swagger/client/ApiClientTest.java
@@ -2,7 +2,7 @@ package io.swagger.client;
 
 import io.swagger.client.auth.*;
 
-import java.util.Map;
+import java.util.*;
 
 import org.junit.*;
 import static org.junit.Assert.*;
@@ -109,4 +109,85 @@ public class ApiClientTest {
         auth.setApiKey(null);
         auth.setApiKeyPrefix(null);
     }
+
+    @Test
+    public void testParameterToPairsWhenNameIsInvalid() throws Exception {
+        List<Pair> pairs_a = apiClient.parameterToPairs("csv", null, new Integer(1));
+        List<Pair> pairs_b = apiClient.parameterToPairs("csv", "", new Integer(1));
+
+        assertTrue(pairs_a.isEmpty());
+        assertTrue(pairs_b.isEmpty());
+    }
+
+    @Test
+    public void testParameterToPairsWhenValueIsNull() throws Exception {
+        List<Pair> pairs = apiClient.parameterToPairs("csv", "param-a", null);
+
+        assertTrue(pairs.isEmpty());
+    }
+
+    @Test
+    public void testParameterToPairsWhenValueIsEmptyStrings() throws Exception {
+
+        // single empty string
+        List<Pair> pairs = apiClient.parameterToPairs("csv", "param-a", " ");
+        assertEquals(1, pairs.size());
+
+        // list of empty strings
+        List<String> strs = new ArrayList<String>();
+        strs.add(" ");
+        strs.add(" ");
+        strs.add(" ");
+
+        List<Pair> concatStrings = apiClient.parameterToPairs("csv", "param-a", strs);
+
+        assertEquals(1, concatStrings.size());
+        assertFalse(concatStrings.get(0).getValue().isEmpty()); // should contain some delimiters
+    }
+
+    @Test
+    public void testParameterToPairsWhenValueIsNotCollection() throws Exception {
+        String name = "param-a";
+        Integer value = 1;
+
+        List<Pair> pairs = apiClient.parameterToPairs("csv", name, value);
+
+        assertEquals(1, pairs.size());
+        assertEquals(value, Integer.valueOf(pairs.get(0).getValue()));
+    }
+
+    @Test
+    public void testParameterToPairsWhenValueIsCollection() throws Exception {
+        Map<String, String> collectionFormatMap = new HashMap<String, String>();
+        collectionFormatMap.put("csv", ",");
+        collectionFormatMap.put("tsv", "\t");
+        collectionFormatMap.put("ssv", " ");
+        collectionFormatMap.put("pipes", "\\|");
+        collectionFormatMap.put("", ","); // no format, must default to csv
+        collectionFormatMap.put("unknown", ","); // all other formats, must default to csv
+
+        String name = "param-a";
+
+        List<Object> values = new ArrayList<Object>();
+        values.add("value-a");
+        values.add(123);
+        values.add(new Date());
+
+        // check for multi separately
+        List<Pair> multiPairs = apiClient.parameterToPairs("multi", name, values);
+        assertEquals(values.size(), multiPairs.size());
+
+        // all other formats
+        for (String collectionFormat : collectionFormatMap.keySet()) {
+            List<Pair> pairs = apiClient.parameterToPairs(collectionFormat, name, values);
+
+            assertEquals(1, pairs.size());
+
+            String delimiter = collectionFormatMap.get(collectionFormat);
+            String[] pairValueSplit = pairs.get(0).getValue().split(delimiter);
+
+            // must equal input values
+            assertEquals(values.size(), pairValueSplit.length);
+        }
+    }
 }
diff --git a/samples/client/petstore/java/src/test/java/io/swagger/client/auth/ApiKeyAuthTest.java b/samples/client/petstore/java/src/test/java/io/swagger/client/auth/ApiKeyAuthTest.java
index 0a0bf57da61..5bdb4fb78fb 100644
--- a/samples/client/petstore/java/src/test/java/io/swagger/client/auth/ApiKeyAuthTest.java
+++ b/samples/client/petstore/java/src/test/java/io/swagger/client/auth/ApiKeyAuthTest.java
@@ -1,8 +1,11 @@
 package io.swagger.client.auth;
 
 import java.util.HashMap;
+import java.util.ArrayList;
 import java.util.Map;
+import java.util.List;
 
+import io.swagger.client.Pair;
 import org.junit.*;
 import static org.junit.Assert.*;
 
@@ -10,7 +13,7 @@ import static org.junit.Assert.*;
 public class ApiKeyAuthTest {
     @Test
     public void testApplyToParamsInQuery() {
-        Map<String, String> queryParams = new HashMap<String, String>();
+        List<Pair> queryParams = new ArrayList<Pair>();
         Map<String, String> headerParams = new HashMap<String, String>();
 
         ApiKeyAuth auth = new ApiKeyAuth("query", "api_key");
@@ -18,14 +21,17 @@ public class ApiKeyAuthTest {
         auth.applyToParams(queryParams, headerParams);
 
         assertEquals(1, queryParams.size());
-        assertEquals("my-api-key", queryParams.get("api_key"));
+        for (Pair queryParam : queryParams) {
+            assertEquals("my-api-key", queryParam.getValue());
+        }
+
         // no changes to header parameters
         assertEquals(0, headerParams.size());
     }
 
     @Test
     public void testApplyToParamsInHeaderWithPrefix() {
-        Map<String, String> queryParams = new HashMap<String, String>();
+            List<Pair> queryParams = new ArrayList<Pair>();
         Map<String, String> headerParams = new HashMap<String, String>();
 
         ApiKeyAuth auth = new ApiKeyAuth("header", "X-API-TOKEN");
diff --git a/samples/client/petstore/java/src/test/java/io/swagger/client/auth/HttpBasicAuthTest.java b/samples/client/petstore/java/src/test/java/io/swagger/client/auth/HttpBasicAuthTest.java
index a621cb60e48..52c5497ba83 100644
--- a/samples/client/petstore/java/src/test/java/io/swagger/client/auth/HttpBasicAuthTest.java
+++ b/samples/client/petstore/java/src/test/java/io/swagger/client/auth/HttpBasicAuthTest.java
@@ -1,8 +1,11 @@
 package io.swagger.client.auth;
 
 import java.util.HashMap;
+import java.util.ArrayList;
 import java.util.Map;
+import java.util.List;
 
+import io.swagger.client.Pair;
 import org.junit.*;
 import static org.junit.Assert.*;
 
@@ -17,7 +20,7 @@ public class HttpBasicAuthTest {
 
     @Test
     public void testApplyToParams() {
-        Map<String, String> queryParams = new HashMap<String, String>();
+        List<Pair> queryParams = new ArrayList<Pair>();
         Map<String, String> headerParams = new HashMap<String, String>();
 
         auth.setUsername("my-username");