learning java AWT 画图

import javax.swing.*;
import java.awt.*;
import java.util.Random;

public class SimpleDraw {
    private final String RECT_SHAPE = "rect";
    private final String OVAL_SHAPE = "oval";
    private Frame f = new Frame("Test");

    private JButton rect = new JButton("绘制矩行");
    private JButton oval = new JButton("绘制圆形");

    private MyCanvas drawArea = new MyCanvas();
    private String shape = "";
    public void init(){

        var p = new Panel();
        rect.addActionListener(e -> {
            shape = RECT_SHAPE;
            drawArea.repaint();
        });

        oval.addActionListener(e -> {
            shape = OVAL_SHAPE;
            drawArea.repaint();
        });

        p.add(rect);
        p.add(oval);
        drawArea.setPreferredSize(new Dimension(250,180));

        f.add(drawArea);
        f.add(p, BorderLayout.SOUTH);

        f.pack();
        f.setVisible(true);

    }
    public static void main(String[] args) {
        new SimpleDraw().init();
    }

    class MyCanvas extends Canvas{
        public void paint(Graphics g){
            var rand = new Random();
            if(shape.equals(RECT_SHAPE)){
                g.setColor(new Color(200,100,80));
                g.drawRect(rand.nextInt(200),
                        rand.nextInt(120),40,60);
            }

            if(shape.equals(OVAL_SHAPE)){
                g.setColor(new Color(80,100,200));
                g.fillOval(rand.nextInt(200),
                rand.nextInt(120),40,50);
            }
        }

    }
}

output: