draw_view.java

public class CanvasView extends View {

    private Paint paint = new Paint();

    public CanvasView(Context context) {
        super(context);
    }

    public CanvasView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
    }

    public CanvasView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    public void draw(Canvas canvas) {
        super.draw(canvas);
        paint.setStyle(Paint.Style.FILL);
        canvas.drawCircle(getRootView().getWidth() / 2, getRootView().getHeight() / 2, 50, paint);
    }
}