我有一个主窗口JFrame，两个JButton和两个JInternalFrame，如何实现：按钮按下时，如果相应的子窗口不存在则创建一个子窗口，如果子窗口存在则把子窗口设置到最前。谢谢！

我的代码：

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import java.util.*;

//主窗口
public class MainFrame extends JFrame {

        //定义菜单栏
        JMenuBar menuBar = new JMenuBar();
        JMenu menuFile= new JMenu("File");
        JMenuItem menuIn = new JMenuItem("窗口1");
        JMenuItem menuOut = new JMenuItem("窗口2");
        JMenuItem menuExit = new JMenuItem("退出");

    //定义内部窗口容器
        static JDesktopPane desktop = new JDesktopPane();
        
        //构造函数
        public MainFrame() {
               
                super("主窗口");

                //设置菜单栏
                menuFile.add(menuIn);
        menuFile.add(menuOut);
        menuFile.add(menuExit);
                menuBar.add(menuFile);
                this.setJMenuBar(menuBar);
                this.getContentPane().add(desktop);
               
                //定义菜单监听
                menuIn.addActionListener(new InnerFrame1());
                menuOut.addActionListener(new InnerFrame2());
                menuExit.addActionListener(new ActionListener() {
                        public void actionPerformed(ActionEvent e) {
                                System.exit(0);
                        }
                });
               
                //设置窗体属性
                this.setSize(500,500);
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        Dimension frameSize = this.getSize();
        if (frameSize.height > screenSize.height) {
                frameSize.height = screenSize.height;
        }
        if (frameSize.width > screenSize.width) {
                frameSize.width = screenSize.width;
        }
        this.setLocation( (screenSize.width - frameSize.width) / 2,
                      (screenSize.height - frameSize.height) / 2);
        this.setVisible(true);
               
                //定义窗口关闭
                this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        }
               
        public static void main(String[] args) {
                MainFrame mainFrame = new MainFrame();
        }
}

//窗口1
class InnerFrame1 extends JInternalFrame implements ActionListener{
        
        //构造函数
        public InnerFrame1() {
                //设置窗体属性
                super("窗口1",true,true,true,true);
                this.setSize(500,500);
                this.setLayout(null);

        }
        
        //定义响应菜单事件
        public void actionPerformed(ActionEvent e) {
//                if (i==0){
//                        i++;
                   try {
                           InnerFrame1 Frame1 = new InnerFrame1();
                           MainFrame.desktop.add(Frame1);
                           Frame1.setMaximum(true);
                           Frame1.show();
                   } catch (Exception eg) {
                           eg.printStackTrace();
                           }
//                }
//                this.toFront();
        }
}

//窗口2
class InnerFrame2 extends JInternalFrame implements ActionListener{
        
        //构造函数
        public InnerFrame2() {
                //设置窗体属性
                super("窗口2",true,true,true,true);
                this.setSize(500,500);
                this.setLayout(null);
        }
        
        //定义响应菜单事件
        public void actionPerformed(ActionEvent e) {
//                if (i==0){
//                        i++;
                   try {
                           InnerFrame2 Frame2 = new InnerFrame2();
                           MainFrame.desktop.add(Frame2);
                           Frame2.setMaximum(true);
                           Frame2.show();
                   } catch (Exception eg) {
                           eg.printStackTrace();
                           }
//                }
//                this.toFront();
        }
}