《Java程序设计》147页 5.5练习题 第8题

题目：在Windows操作系统中有一个“画图”程序，可以用来绘制各种各样的图形。请编写一个程序，实现该软件的基本功能，只要实现画直线、椭圆和矩形这3个功能即可。

import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
public class PaintDemo extends JFrame{  
    public static void main(String args[]){          
    	PaintDemo demo = new PaintDemo();  
        demo.go();       
    }  
    public void go(){            
        this.setTitle("画图");      	
        this.setSize(600,500);            
        this.setLayout(new FlowLayout());
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setLocationRelativeTo(null);
        this.setVisible(true);   
        //按钮组
        ButtonGroup group = new ButtonGroup();
        //单选按钮  
        JRadioButton line = new JRadioButton("直线");
        //设置按钮的动作命令  
        line.setActionCommand("line");  
        //默认选中直线  
        line.setSelected(true);           
        JRadioButton rect = new JRadioButton("矩形");  
        rect.setActionCommand("rect");           
        JRadioButton oval = new JRadioButton("椭圆");  
        oval.setActionCommand("oval");            
        group.add(line);  
        group.add(rect);  
        group.add(oval);          
        this.add(line);  
        this.add(rect);  
        this.add(oval);            
        Graphics g = this.getGraphics();  
        MyListener listener = new MyListener(g,group);   
        this.addMouseListener(listener);   
    }  
}  
class MyListener implements MouseListener {
    int startX, startY, endX, endY;   
    Graphics g;
    ButtonGroup group;
    public MyListener(Graphics g,ButtonGroup group) {  
        this.g = g;  
        this.group = group;  
    }    
    // 按下  
    public void mousePressed(MouseEvent e) {  
        startX = e.getX();  
        startY = e.getY();  
    }  
    // 释放  
    public void mouseReleased(MouseEvent e) {  
    	endX = e.getX();  
    	endY = e.getY();
    	String str = group.getSelection().getActionCommand();
        if ("line".equals(str)) {  
            g.drawLine(startX, startY, endX, endY);  
        } else if (str.equals("rect")) {  
            g.drawRect(startX, startY, endX - startX, endY - startY);  
        } else{  
            g.drawOval(startX, startY, endX - startX, endY - startY);  
        } 
    }  
    // 进入  
    public void mouseEntered(MouseEvent e) {    
    }   
    // 离开  
    public void mouseExited(MouseEvent e) {   
    }   
    // 点击  
    public void mouseClicked(MouseEvent e) {  
    }  
}  

运行效果：