为什么应用启动后立即消失

Question

I am creating basic app that is drawing squares in the screen. Everytime after launching my app disappears instantly. Here is my code:

I have deleted auto created methods such as onCreateOptionsMenuSelected in MainActivity

public class MainActivity extends AppCompatActivity {
public static int WIDTH;
{
    WIDTH = getWindowManager().getDefaultDisplay().getWidth();
}
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    View customView=new CustomView(this);
    customView.setBackgroundColor(Color.BLACK);
    setContentView(customView);
}
}

And the next class

public class CustomView extends View {
public static final int ELEMENTS=15;
private Paint paint=new Paint();
int x = 50;
int y = 50;

/////////////////////////////////////////////////////////
public CustomView(Context context){
    super(context);
}
////////////////////////////////////////////////////////////////////
@Override
public void onDraw(Canvas canvas){
    paint.setColor(Color.BLACK);
    canvas.drawRect(30, 30, 80, 80, paint);  //some arbitrary numbers

}
}

Answer 1

replace:

public static int WIDTH;{
WIDTH = getWindowManager().getDefaultDisplay().getWidth();
}

with:

public int width(){
    DisplayMetrics displayMetrics = new DisplayMetrics();
    getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);
    int width = displayMetrics.widthPixels;
    return width;
}

Or remove it.

and also try changing either you paint or customView background color because they are both set to BLACK