I want to make one screen function a different file.

//import is omitted.
public class MainActivity extensions AppCompatActivity implements View.OnClickListener{
    private EditText text;

    @ Override
    protected void onCreate (Bundle state) {
        super.onCreate(state);
        setContentView (R.layout.main);
        // Add FrameLayout, etc. to the layout and replace it with the Fragment that added ListView.
        // Here we use Button and EditText.Replace it with GridView and think about it.
        etext=(EditText)findViewById(R.id.etext);
        Button btn=(Button) findViewById (R.id.button);
        btn.setOnClickListener(this);
    }

    @ Override
    public void onClick (View v) {
        // Invoke the onItemAdd method implemented in Fragment.
        if(listener!=null){
            listener.onItemAdd(etext.getText().toString);
        }
    }

    private onItemAddListener listener;

    public void setOnItemAddListener(OnItemAddListener){
        This.listener=listener;
    }

    public interface onItemAddListener{
        // Specifies the type of value that you want to pass to Fragment.
        void onAddItemAdd(String label);
    }
}

//import is omitted.
public class ListFragment extensions Fragment implements MainActivity.OnItemAddListener{
    public ListFragment() {/*empty*/}

    private TextView textView;

    @ Override
publicView onCreateView(LayoutInflater inflater, ViewGroup container, Bundle state) {
        View v=inflater.inflate(R.layout.list, container, false);

        textView=(TextView) v.findViewById (R.id.textView);
        MainActivity=(MainActivity) getActivity();
        activity.setOnItemAddListener(this);

        return v;
    }

    @ Override
    public void onItemAdd(String label) {
        textView.setText(textView.getText().toString()+label);
    }