public class MainActivity extends AppCompatActivity {

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

        FragmentManager fragmentManager = getFragmentManager();
        FragmentTransaction transaction = fragmentManager.beginTransaction();
        transaction.add(R.id.main_layout,myfragment,"key");
        transaction.commit();
    }
}