如何在android studio中初始化画布？

我试图在现有类上绘制一个矩形。我看到的很多教程都创建了一个新类。但是，我得到一个错误，说画布没有初始化。如何初始化画布？谢谢

 @Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_display_message);
    // Get the message from the intent
    Intent intent = getIntent();
    Context context;
    Canvas canvas;
    Rect ourRect = new Rect();
    ourRect.set(0,0,canvas.getWidth(),canvas.getHeight()/2);

    Paint blue = new Paint();
    blue.setColor(Color.BLUE);
    blue.setStyle(Paint.Style.FILL);

    canvas.drawRect(ourRect,blue);


}

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初始化canvas变量，如下所示

   Canvas canvas = new Canvas();