findViewById can't find the view of a programmatically generated view

Question

When I call findViewById() to get a button (in the form of an ImageView), it returns null. I have looked over these suggestions and I can tell that

• findViewById() is called after setContentView().
• There is only one layout: activity_main.xml.
• I am not using ExpandableListView.
• Both findViewById() and topbar.findViewById() return null.

What else might I have overlooked?

activity_main.xml

<LinearLayout>
 <GridView
   android:id="@+id/topbar_grid">
 </GridView>
</LinearLayout>

Mainactivity.java

public class MainActivity extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        initializeTopBar();
        initializeMenu();
    }

    private void initializeTopBar() {
        GridView topbar = (GridView) findViewById(R.id.topbar_grid);
        TopBarAdapter topBarAdapter = new TopBarAdapter(this);
        topbar.setAdapter(topBarAdapter);

        ImageView playButton = (ImageView) findViewById(topBarAdapter.playButtonId);
        // Debugging shows playButton is null here
        // Removing the below statement results in the app 
        // being displayed just fine
        playButton.setOnClickListener(new Button.OnClickListener(){
            public void onClick(View v){
                onPlayButtonClick();
            }
        }); 
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }
}

TopBarAdapter.java

public class TopBarAdapter extends BaseAdapter {
    private Context context;
    private View[] views = new View[2];

    public int playButtonId = 2000;
    public int trackLabelId = 2001;

    public TopBarAdapter(Context c){
        context = c;
        createTrackLabel();
        createPlayButton();
    }

    private void createPlayButton(){
        ImageView playButton = new ImageView(context);
        playButton.setImageResource(R.drawable.play_icon);
        playButton.setScaleType(ScaleType.CENTER_CROP);
        playButton.setLayoutParams(new GridView.LayoutParams(50, 50));
        playButton.setId(playButtonId);
        views[1] = playButton;
    }




    @Override
    public int getCount() {
        return views.length;
    }

    @Override
    public Object getItem(int position) {
        return views[position];
    }

    @Override
    public long getItemId(int position) {
        return 0;
    }

    @Override
    public View getView(int position, View convertView, ViewGroup parent) {
        return views[position];
    }
}

Answer 1

you should set the OnItemClickListener on your topbar:

topbar.setOnItemClickListener(new OnItemClickListener() {

    @Override
    public void onItemClick(AdapterView<?> parent, View view, 
            int position, long id) {

    }
});

Answer 2

Is the playButton added somewhere to your layout? Where it comes from? The best way to use findViewById method is to rely on Android-generater ids, so instead

     Button playButton = (Button) findViewById(topBarAdapter.playButtonId);

You should rather rely on

Button playButton = (Button) findViewById(R.id.playButtonId);

given that <Button/> with id playButtonId is actually a part of your current view's layout specified by R.layout.activity_main . When looking for topBarAdapter children's id's you should rather call

Button playButton = (Button) topBarAdapter.findViewById(R.id.playButtonId);