added okhttp4-gson ibrary for java generator

src/main/java/io/swagger/codegen/v3/generators/java/JavaClientCodegen.java

@@ -91,6 +91,7 @@ public JavaClientCodegen() {
         supportedLibraries.put("feign", "HTTP client: OpenFeign 9.4.0. JSON processing: Jackson 2.10.1");
         supportedLibraries.put("jersey2", "HTTP client: Jersey client 2.26. JSON processing: Jackson 2.10.1");
         supportedLibraries.put("okhttp-gson", "HTTP client: OkHttp 2.7.5. JSON processing: Gson 2.8.1. Enable Parcelable models on Android using '-DparcelableModel=true'. Enable gzip request encoding using '-DuseGzipFeature=true'.");
+        supportedLibraries.put("okhttp4-gson", "HTTP client: OkHttp 4.10.0. JSON processing: Gson 2.10.1. Enable Parcelable models on Android using '-DparcelableModel=true'. Enable gzip request encoding using '-DuseGzipFeature=true'.");
         supportedLibraries.put(RETROFIT_1, "HTTP client: OkHttp 2.7.5. JSON processing: Gson 2.3.1 (Retrofit 1.9.0). IMPORTANT NOTE: retrofit1.x is no longer actively maintained so please upgrade to 'retrofit2' instead.");
         supportedLibraries.put(RETROFIT_2, "HTTP client: OkHttp 3.8.0. JSON processing: Gson 2.6.1 (Retrofit 2.3.0). Enable the RxJava adapter using '-DuseRxJava[2]=true'. (RxJava 1.x or 2.x)");
         supportedLibraries.put("resttemplate", "HTTP client: Spring RestTemplate 4.3.9-RELEASE. JSON processing: Jackson 2.9.9");
@@ -231,7 +232,7 @@ public void processOpts() {
             additionalProperties.put("jackson", "true");
             supportingFiles.add(new SupportingFile("ParamExpander.mustache", invokerFolder, "ParamExpander.java"));
             supportingFiles.add(new SupportingFile("EncodingUtils.mustache", invokerFolder, "EncodingUtils.java"));
-        } else if ("okhttp-gson".equals(getLibrary()) || StringUtils.isEmpty(getLibrary())) {
+        } else if ("okhttp-gson".equals(getLibrary()) || "okhttp4-gson".equals(getLibrary()) || StringUtils.isEmpty(getLibrary())) {
             // the "okhttp-gson" library template requires "ApiCallback.mustache" for async call
             supportingFiles.add(new SupportingFile("ApiCallback.mustache", invokerFolder, "ApiCallback.java"));
             supportingFiles.add(new SupportingFile("ApiResponse.mustache", invokerFolder, "ApiResponse.java"));

src/main/resources/handlebars/Java/libraries/okhttp4-gson/ApiCallback.mustache

@@ -0,0 +1,51 @@
+{{>licenseInfo}}
+
+package {{invokerPackage}};
+
+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 downlond processing.
+     *
+     * @param bytesRead bytes Read
+     * @param contentLength content lenngth of the response
+     * @param done Read end
+     */
+    void onDownloadProgress(long bytesRead, long contentLength, boolean done);
+}