java 编写一个 GUI 程序，界面上包含一个按钮，点击按钮后在界面上画一个 矩形。

以下是一个简单的Java GUI程序，界面上有一个按钮，点击按钮后会在界面上画一个矩形。

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class DrawRectangle extends JPanel implements ActionListener{
    private JButton button;
    private boolean draw = false;
    
    public DrawRectangle(){
        setLayout(null);
        button = new JButton("Draw Rectangle");
        button.setBounds(50, 50, 150, 30);
        add(button);
        button.addActionListener(this);
    }
    
    protected void paintComponent(Graphics g){
        super.paintComponent(g);
        if(draw == true){
            g.setColor(Color.RED);
            g.drawRect(100, 100, 100, 100);
        }
    }
    
    public void actionPerformed(ActionEvent event){
        if(event.getSource() == button){
            draw = true;
            repaint();
        }
    }
    
    public static void main(String[] args){
        JFrame frame = new JFrame("Draw Rectangle");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(300, 300);
        frame.setLocationRelativeTo(null);
        frame.setContentPane(new DrawRectangle());
        frame.setVisible(true);
    }
}

运行该程序后，点击按钮即可在界面上画出一个红色的矩形。