OkHttpClient.Builder httpClient = new OkHttpClient.Builder();httpClient.addInterceptor(new Interceptor() {    @Override    public Response intercept(Chain chain) throws IOException {        Request request = chain.request().newBuilder().addHeader("parameter", "value").build();        return chain.proceed(request);    }});Retrofit retrofit = new Retrofit.Builder().addConverterFactory(GsonConverterFactory.create()).baseUrl(url).client(httpClient.build()).build();

The Latest Retrofit Version HERE -> 2.1.0.

lambda version:

  builder.addInterceptor(chain -> {    Request request = chain.request().newBuilder().addHeader("key", "value").build();    return chain.proceed(request);  });

ugly long version:

  builder.addInterceptor(new Interceptor() {    @Override public Response intercept(Chain chain) throws IOException {      Request request = chain.request().newBuilder().addHeader("key", "value").build();      return chain.proceed(request);    }  });

full version:

class Factory {public static APIService create(Context context) {  OkHttpClient.Builder builder = new OkHttpClient().newBuilder();  builder.readTimeout(10, TimeUnit.SECONDS);  builder.connectTimeout(5, TimeUnit.SECONDS);  if (BuildConfig.DEBUG) {    HttpLoggingInterceptor interceptor = new HttpLoggingInterceptor();    interceptor.setLevel(HttpLoggingInterceptor.Level.BASIC);    builder.addInterceptor(interceptor);  }  builder.addInterceptor(chain -> {    Request request = chain.request().newBuilder().addHeader("key", "value").build();    return chain.proceed(request);  });  builder.addInterceptor(new UnauthorisedInterceptor(context));  OkHttpClient client = builder.build();  Retrofit retrofit =      new Retrofit.Builder().baseUrl(APIService.ENDPOINT).client(client).addConverterFactory(GsonConverterFactory.create()).addCallAdapterFactory(RxJavaCallAdapterFactory.create()).build();  return retrofit.create(APIService.class);  }}

gradle file (you need to add the logging interceptor if you plan to use it):

  //----- Retrofit  compile 'com.squareup.retrofit2:retrofit:2.1.0'  compile "com.squareup.retrofit2:converter-gson:2.1.0"  compile "com.squareup.retrofit2:adapter-rxjava:2.1.0"  compile 'com.squareup.okhttp3:logging-interceptor:3.4.0'

Try this type header for Retrofit 1.9 and 2.0. For Json Content Type.

@Headers({"Accept: application/json"})@POST("user/classes")Call<playlist> addToPlaylist(@Body PlaylistParm parm);

You can add many more headers i.e

@Headers({        "Accept: application/json",        "User-Agent: Your-App-Name",        "Cache-Control: max-age=640000"    })

Dynamically Add to headers:

@POST("user/classes")Call<ResponseModel> addToPlaylist(@Header("Content-Type") String content_type, @Body RequestModel req);

Call you method i.e

mAPI.addToPlayList("application/json", playListParam);

Or

Want to pass everytime then Create HttpClient object with http Interceptor:

OkHttpClient httpClient = new OkHttpClient();        httpClient.networkInterceptors().add(new Interceptor() {            @Override            public com.squareup.okhttp.Response intercept(Chain chain) throws IOException {                Request.Builder requestBuilder = chain.request().newBuilder();                requestBuilder.header("Content-Type", "application/json");                return chain.proceed(requestBuilder.build());            }        });

Then add to retrofit object

Retrofit retrofit = new Retrofit.Builder().baseUrl(BASE_URL).client(httpClient).build();

UPDATE if you are using Kotlin remove the { } else it will not work