How to draw something on Android layout?
<p>This is the activity</p>

<pre><code>public class MainActivity extends AppCompatActivity {

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

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

}
</code></pre>

<p>I know I can create a class</p>

<pre><code>public class DrawView extends View {
    Paint paint = new Paint();

    public DrawView(Context context) {
        super(context);
        paint.setColor(Color.BLACK);
    }

    @Override
    public void onDraw(Canvas canvas) {
        canvas.drawLine(0, 0, 20, 20, paint);
        canvas.drawLine(20, 0, 0, 20, paint);
    }
}
</code></pre>

<p>And use it in activity</p>

<pre><code>@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    drawView = new DrawView(this);
    drawView.setBackgroundColor(Color.WHITE);
    setContentView(drawView);

}
</code></pre>

<p>But it must replace my original layout <code>setContentView(R.layout.activity_main);</code></p>

<p>If I use layout, I cannot draw something, if I draw something, I cannot use layout</p>

<p>How can I keep both?</p>