public void addUser(User user){
        Call<User> call=userService.addUser(user);
        call.enqueue(new Callback<User>() {
            @Override
            public void onResponse(Call<User> call, Response<User> response) {
                if (response.isSuccessful()){
                    Toast.makeText(MainActivity.this, "Add Successfully", Toast.LENGTH_SHORT).show();
                }
            }

            @Override
            public void onFailure(Call<User> call, Throwable t) {
                Log.e("ERR", t.getMessage() );
            }
        });
    }