画图,橡皮

import javax.swing.*;
import java.applet.Applet;
import java.awt.event.*;
import java.awt.*;
import java.util.Vector;
public class Exec76 extends Applet {
    Graphics2D g2 ;
    Vector<Point> points = new Vector<Point>();
    Polygon po = new Polygon();
    Boolean eraser = false;
    public JButton bt;
    int x=0,y=0;
    int w = 20;
    public Exec76() {
        addMouseMotionListener(new MLis());
         bt = new JButton("Draw");
        bt.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e ) {
                if(!eraser) { 
                    bt.setText("Eraser");
                    eraser = true;
                    repaint();
                }
                else {
                    bt.setText("Draw");
                    eraser = false;
                    repaint();
                }
            }
        });
        
        
    }
    public void init(){
        addMouseMotionListener(new MLis());
         bt = new JButton("Draw");
        bt.addActionListener(new ActionListener(){
            public void actionPerformed(ActionEvent e ) {
                if(!eraser) { 
                    bt.setText("Eraser");
                    eraser = true;
                    repaint();
                }
                else {
                    bt.setText("Draw");
                    eraser = false;
                    repaint();
                }
            }
        });
        add(bt);
    }
    public void paint(Graphics g) {
        g2 = (Graphics2D)g;
        if (eraser) { 
            Rectangle r = new Rectangle(x,y,w,w);
            g2.setColor(Color.BLUE);
            g2.fill(r);
            for(int i=0;i<points.size();i++) {
                if (r.contains(points.get(i))) points.remove(i);
            }
        }
        g2.setColor(Color.BLACK);
        for(int i=0;i<points.size();i++) {
            g2.fillOval((int)points.get(i).getX(), (int)points.get(i).getY(), 4, 4);
        }

    }

    class MLis extends MouseAdapter {
        public void mouseDragged(MouseEvent e) {
            if (eraser) { x = e.getX(); y = e.getY();repaint();}
            else {
                points.add(new Point(e.getX(),e.getY()));
                repaint();
            }
        }
    }
    
    public static void main(String[] args) {
        JFrame f = new JFrame("Exec76");
        f.setSize(400,400);
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setLocationRelativeTo(null);
        Exec76 panel = new Exec76();
        f.add(panel.bt,BorderLayout.NORTH);
        f.add(panel);
        f.setVisible(true);
        
    }

}