一个五子棋程序

/*
*  filename: Game.java
*
*  author: jellen
*
*/
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import java.awt.geom.*;
import java.util.*;

class PaintPanel extends JPanel {
  ArrayList Items = new ArrayList();

  public PaintPanel() {
    setLayout(new BorderLayout());
    ButtonPanel buttonPanel = new ButtonPanel();
    add(buttonPanel, BorderLayout.SOUTH);
    addMouseListener(new MouseHandler());
  }

  public void paintComponent(Graphics g) {
    int startX = 50;
    int startY = 50;
    boolean isMy = false;
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D)g;

    for(int i=0; i<16; i++) {
      g2.draw(new Line2D.Double(startX, startY+20*i, startX+300, startY+20*i));
      g2.draw(new Line2D.Double(startX+20*i, startY, startX+20*i, startY+300));
    }
    for(int i=0; i<Items.size(); i++) {
      isMy = !isMy;
      if(isMy)
        g2.setColor(Color.BLACK);
      else
        g2.setColor(Color.WHITE);
      g2.fill((Ellipse2D)Items.get(i));
    }
  }
  void myRepaint() {
    repaint();
  }
  private class MouseHandler extends MouseAdapter {
    public void mousePressed(MouseEvent event) {
      int x = event.getX();
      int y = event.getY();
      paintItem(x, y);
      repaint();
    }
    void paintItem(int x, int y) {
      if(x > 50 && x < 350 && y >50 && y < 350) {
        int X = x / 20;
        int Y = y / 20;
        int centerX = X*20 + 10;
        int centerY = Y*20 + 10;
        Ellipse2D ellipse = new Ellipse2D.Double();
        ellipse.setFrameFromCenter(centerX, centerY, centerX+8, centerY+8);
        Items.add(ellipse);
      }
    }
  }
  private class ButtonPanel extends JPanel {
    public ButtonPanel() {
      JButton reset = new JButton("Reset");
      add(reset);
      JButton quit = new JButton("Quit");
      add(quit);

      ResetEvent listenerR = new ResetEvent();
      reset.addMouseListener(listenerR);
      QuitEvent listenerQ = new QuitEvent();
      quit.addMouseListener(listenerQ);
    }

    private class QuitEvent
        extends MouseAdapter {
      public void mouseClicked(MouseEvent event) {
        System.exit(1);
      }
    }

    private class ResetEvent
        extends MouseAdapter {
      public void mouseClicked(MouseEvent event) {
        Items.clear();
        myRepaint();
      }
    }
  }
}

class GameFrame extends JFrame {
  public GameFrame() {
    setTitle("A Little Game");
    setSize(400, 500);
    setResizable(false);

    PaintPanel panel = new PaintPanel();
    getContentPane().add(panel);
  }
} public class Game {
  public static void main(String[] args) {
    GameFrame frame = new GameFrame();
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.show();
  }
}