Java随鼠标画曲线

package Rong;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;

public class DrawCurve implements MouseMotionListener,ActionListener {
 static int ox, oy;
    static intflag = 0;
    staticJFrame f;
 public static void main(String arg[]) {
  f = new JFrame("鼠标画曲线");
  Container p = f.getContentPane();
  p.setLayout(new FlowLayout());
  JButton red = new JButton("红色");
  JButton blue = new JButton("蓝色");
  JButton green = new JButton("绿色");
  red.addActionListener(new DrawCurve());
  blue.addActionListener(new DrawCurve());
  green.addActionListener(new DrawCurve());
  p.addMouseMotionListener(new DrawCurve());
  p.add(red);
  p.add(blue);
  p.add(green);
  f.setSize(400, 400);
  f.setVisible(true);
  Dimensiondim=Toolkit.getDefaultToolkit().getScreenSize();
 f.setLocation((dim.width-f.getWidth())/2,(dim.height-f.getHeight())/2);
 f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 }

 public void actionPerformed(ActionEvent e){
  if (e.getActionCommand().equals("红色")) {
   flag=1;
  }
  else if (e.getActionCommand().equals("蓝色")){
   flag=2;
  }
  else if(e.getActionCommand().equals("绿色")){
   flag=3;
  }
 }

 public void mouseDragged(MouseEvent e) {
  Container c = (Container) e.getSource();
  Graphics g = c.getGraphics();
  if (ox >= 0) {
   if(flag==0){
   g.setColor(Color.black);
   g.drawLine(ox, oy, e.getX(), e.getY());
   }
   else if(flag == 1) {
   g.setColor(Color.red);
   g.drawLine(ox, oy, e.getX(), e.getY());
   }
   else if(flag==2){
   g.setColor(Color.blue);
   g.drawLine(ox, oy, e.getX(), e.getY());
   }
   else if(flag==3){
   g.setColor(Color.green);
   g.drawLine(ox, oy, e.getX(), e.getY());
   }
  }
  ox = e.getX();
  oy = e.getY();
 }

 public void mouseMoved(MouseEvent e) {
  ox = -1;
  oy = -1;
 }
}