再窗口上切换图形的时候，已经画好的图形也随之转换过来。
没有能够达到3种图形共存。怎么修改比较好呢？
就是想画直线的时候出直线，想画圆的时候出圆，但它们彼此共存再窗口上。
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import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;
import java.awt.event.*;
import java.util.*;

public class Test{
    public static void main(String[] args){
    MyFrame frame = new MyFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setVisible(true);
    }
}

class MyFrame extends JFrame implements ActionListener{

     MyPanel panel = new MyPanel();
   
    JToolBar t;
    JButton obj1;
    JButton obj2;
    JButton obj3;

    public MyFrame(){

    obj1 = new JButton("Line");
    obj2 = new JButton("yuan");
    obj3 = new JButton("juxing");
    t = new JToolBar();
    obj1.addActionListener(this);
    obj2.addActionListener(this);
    obj3.addActionListener(this);

    t = new JToolBar();
    t.add(obj1);
    t.add(obj2);
    t.add(obj3);


    setTitle("DrawTest");
    setSize(W,H);

    Container contentPane = getContentPane();

    contentPane.add(t,BorderLayout.NORTH);
    contentPane.add(panel,BorderLayout.CENTER);
    }

    public void actionPerformed(ActionEvent e){
    if(e.getSource()==obj1)
        panel.shape=0;//直線
    if(e.getSource()==obj2)
        panel.shape=1;//円
    if(e.getSource()==obj3)
        panel.shape=2;//長方形
    }
   
    public static final int W = 400;
    public static final int H = 400;
}


class MyPanel extends JPanel implements MouseListener{

  
    private ArrayList<Point> pointList = new ArrayList<Point>();


    int shape = -1;
    int x1 = 0,y1 = 0;
    int x2 = 0,y2 = 0;

    MyPanel(){
    addMouseListener(this);
    }
   
    public void mousePressed(MouseEvent e){

    x1 = e.getX();
    y1 = e.getY();
    pointList.add(new Point(x1,y1));

    }
    public void mouseReleased(MouseEvent e){

    x2 = e.getX();
    y2 = e.getY();
    pointList.add(new Point(x2,y2));
    repaint();

    }
   
    public void mouseClicked(MouseEvent e){
    }
   
    public void mouseMoved(MouseEvent e){
    }
   
    public void mouseEntered(MouseEvent e){
    }
   
    public void mouseExited(MouseEvent e){
    }
   
   

    public void paintComponent(Graphics g){
    super.paintComponent(g);

    for(int i =0 ;i<pointList.size()-1;i=i+2){
        Point p1 = pointList.get(i);
        Point p2 = pointList.get(i+1);
        
        switch(shape){
        
        case 0:        
        g.drawLine(p1.x,p1.y,p2.x,p2.y);
        
        break;
        case 1:
        
        int width1  = p2.x - p1.x;
        int height1 = p2.y - p1.y;
        g.drawOval(p1.x,p1.y,width1,height1);
        break;
        case 2:
        int width2  = p2.x - p1.x;
        int height2 = p2.y - p1.y;
        g.drawRect(p1.x,p1.y,width2,height2);
        break;
        
        default:
        System.out.println("please once again!");
        break;
        }   
    }
    }
}

谢谢，我已经写出来了。你的也可以拿出参考。