[android] Button button = findViewById(R.id.button) always resolves to null in Android Studio

I'm new to Android development and Android Studio, so pardon my ignorance.

findViewById of a button I added always resolves to null. Hence if I try to setonClickListener it fails the whole Activity.

MainActivity:

public class MainActivity extends ActionBarActivity {

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

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                    .add(R.id.container, new PlaceholderFragment())
                    .commit();
        }

        Button buttonClick = (Button) findViewById(R.id.button);
        buttonClick.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                onButtonClick((Button) view);
            }

        });

    }

    public void onButtonClick(Button view) {
        TextView textHello = (TextView) findViewById(R.id.textView);
        textHello.setText("Clicked !!!");
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
    }

}

activity_main.xml

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/container"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.pluralsight.activitylifecycle.MainActivity"
tools:ignore="MergeRootFrame" />

fragment_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="com.pluralsight.activitylifecycle.MainActivity$PlaceholderFragment">

<TextView
    android:text="@string/hello_world"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/textView" />

<Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/buttonClick"
    android:id="@+id/button"
    android:layout_below="@+id/textView"
    android:layout_alignParentRight="true"
    android:layout_alignParentEnd="true"
    android:layout_marginRight="54dp" />

</RelativeLayout>

This question is related to android android-studio

The answer is


The button code should be moved to the PlaceholderFragment() class. There you will call the layout fragment_main.xml in the onCreateView method. Like so

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
        Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.fragment_main, container, false);
    Button buttonClick = (Button) view.findViewById(R.id.button);
    buttonClick.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            onButtonClick((Button) view);
        }

    });

    return view;
}

R.id.button is not part of R.layout.activity_main. How should the activity find it in the content view?

The layout that contains the button is displayed by the Fragment, so you have to get the Button there, in the Fragment.