Callback is a way to inform a class synchronous/ asynchronous when an event occurs. A simple way to create a generic callback is to use interface with a method in which JSON is passed.

So, In this tutorial, I will guide you about how to implement callback in Android. You can use this callback to communicate between classes, fragments or adapters.

Let's see with example:-

public interface CallBacks {

    void callbackObserver(JSONObject obj);

}

Now, we are going to use this callback in our class as follow:-
 

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        loginButtonClicked(new Callbacks() {
            @Override
            public void callbackObserver(JSONObject obj) {
                //Do work after login
            }
        });
    }


    void loginButtonClicked(Callbacks callback){
        //Api request to login or any kind of work
        JSONObject object = new JSONObject();
        try {
            object.put("name", "Jagandeep Singh");
        }catch (JSONException exception){
            exception.printStackTrace();
        }
        callback.callbackObserver(object);
    }
}

In above example, we have passed a callback as a parameter to function and after some task or after an event, we are calling the callback method with some information.