"Expected BEGIN_OBJECT but was STRING at line 1 column 1"

Even without seeing your JSON string you can tell from the error message that it is not the correct structure to be parsed into an instance of your class.

Gson is expecting your JSON string to begin with an object opening brace. e.g.

{

But the string you have passed to it starts with an open quotes

"

Invalid JSON from the server should always be an expected use case. A million things can go wrong during transmission. Gson is a bit tricky, because its error output will give you one problem, and the actual exception you catch will be of a different type.

With all that in mind, the proper fix on the client side is

try{  gson.fromJSON(ad, Ad.class);  //...}catch (IllegalStateException | JsonSyntaxException exception){  //...

If you want to know why the JSON you received from the server is wrong, you can look inside your catch block at the exception. But even if it is your problem, it's not the client's responsibility to fix JSON it is receiving from the internet.

Either way, it is the client's responsibility to decide what to do when it gets bad JSON. Two possibilities are rejecting the JSON and doing nothing, and trying again.

If you are going to try again, I highly recommend setting a flag inside the try / catch block and then responding to that flag outside the try / catch block. Nested try / catch is likely how Gson got us into this mess with our stack trace and exceptions not matching up.

In other words, even though I'll admit it doesn't look very elegant, I would recommend

boolean failed = false;try{  gson.fromJSON(ad, Ad.class);  //...}catch (IllegalStateException | JsonSyntaxException exception){  failed = true;  //...}if (failed){  //...

In Retrofit2, When you want to send your parameters in raw you must use Scalars.

first add this in your gradle:

    compile 'com.squareup.retrofit2:retrofit:2.3.0'    compile 'com.squareup.retrofit2:converter-gson:2.3.0'    compile 'com.squareup.retrofit2:converter-scalars:2.3.0'    public interface ApiInterface {    String URL_BASE = "http://10.157.102.22/rest/";    @Headers("Content-Type: application/json")    @POST("login")    Call<User> getUser(@Body String body);}

my SampleActivity :

   public class SampleActivity extends AppCompatActivity implements Callback<User> {    @Override    protected void onCreate(@Nullable Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_sample);        Retrofit retrofit = new Retrofit.Builder()                .baseUrl(ApiInterface.URL_BASE)                .addConverterFactory(ScalarsConverterFactory.create())                .addConverterFactory(GsonConverterFactory.create())                .build();        ApiInterface apiInterface = retrofit.create(ApiInterface.class);        // prepare call in Retrofit 2.0        try {            JSONObject paramObject = new JSONObject();            paramObject.put("email", "sample@gmail.com");            paramObject.put("pass", "4384984938943");            Call<User> userCall = apiInterface.getUser(paramObject.toString());            userCall.enqueue(this);        } catch (JSONException e) {            e.printStackTrace();        }    }    @Override    public void onResponse(Call<User> call, Response<User> response) {    }    @Override    public void onFailure(Call<User> call, Throwable t) {    }}

Reference: [How to POST raw whole JSON in the body of a Retrofit request?