Add Header Parameter in Retrofit

Try this type header for Retrofit 1.9 and 2.0. For the 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 your method, i.e.,

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

Or

Want to pass every time, then create an HttpClient object with the 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 a Retrofit object

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

If you are using Kotlin, remove the { }. Else it will not work.

You can use the below

 @Headers("user-key: 9900a9720d31dfd5fdb4352700c")
 @GET("api/v2.1/search")
 Call<String> getRestaurantsBySearch(@Query("entity_id") String entity_id, @Query("entity_type") String entity_type, @Query("q") String query);

and

 Call<String> call = endpoint.getRestaurantsBySearch("3","city","cafes");

The above is based in the zomato api which is documented at

Thing to note is the end point change api/v2.1/search and the Header @Headers("user-key: 9900a9720d31dfd5fdb4352700c").

Also check your base url .baseUrl("")

Also i tried the above with a api key i generated and it works and my query was cafes as suggested the zomato documentation.

Note : I hope you have the below

 .addConverterFactory(ScalarsConverterFactory.create()) // for string conversion
 .build();

and the below in build.gradle file

compile group: 'com.squareup.retrofit2', name: 'converter-scalars', version: '2.2.0'

Edit:

You can also pass header with dynamic value as below

@GET("api/v2.1/search")
Call<String> getRestaurantsBySearch(@Query("entity_id") String entity_id, @Query("entity_type") String entity_type, @Query("q") String query,@Header("user-key") String userkey);

And

Call<String> call = endpoint.getRestaurantsBySearch("3","city","cafes","9900a9720d31dfd5fdb4352700c");

As far as i can see you are passing the data in a wrong way. Your method getRestaurantsBySearch is accepting the last two parameter as header field i.e accept and user-key. But while calling the method you are passing headers first. Pass the data as you have declared it in method signature of getRestaurantsBySearch

Please take a look at the response. It clearly shows that the api key you provided is wrong. At first you get the correct api key. Then call the request it will work .