自学的GUI小知识

import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Toolkit;
import java.awt.event.ActionEvent;
import java.awt.event.KeyEvent;
import javax.swing.*;
import javax.swing.event.*;
import java.awt.event.*;

class JMenuFrame extends JFrame implements ActionListener {
private FlowLayout flowLayout;
private JMenuBar fileMenu1,fileMenu2;
private JMenu file1,file2;
private JMenuItem newItem,openItem,closeItem,openFromItem,closeFromItem;
Toolkit tk=Toolkit.getDefaultToolkit();
Dimension d=tk.getScreenSize();
public JMenuFrame(){
setTitle("建立菜单栏");
setSize(300,400);
flowLayout=new FlowLayout(FlowLayout.RIGHT);
setLayout(flowLayout);
setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
center(this);
fileMenu1=new JMenuBar();
fileMenu2=new JMenuBar();
file1=new JMenu("文件");
file2=new JMenu("编辑");
fileMenu1.add(file1);
fileMenu2.add(file2);
newItem=new JMenu("新建");
openFromItem=new JMenuItem("open from file");
openFromItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_O,KeyEvent.ALT_MASK));
openFromItem.addActionListener(new ActionListener(){

public void actionPerformed(ActionEvent e) {
// TODO Auto-generated method stub
JOptionPane.showMessageDialog(null, "打开文件");
}

});
closeFromItem=new JMenuItem("close from file");
newItem.add(openFromItem);
newItem.add(closeFromItem);
openItem=new JMenuItem("open",'o');
closeItem=new JMenuItem("关闭");
closeItem.setAccelerator(KeyStroke.getKeyStroke(KeyEvent.VK_C,KeyEvent.ALT_MASK));
closeItem.addActionListener(this);
file1.add(newItem);
file1.add(openItem);
file1.add(closeItem);
this.add(fileMenu1);
this.add(fileMenu2);


}
private void center(JFrame frame) {
// TODO Auto-generated method stub
int centerWidth=((int)d.getWidth()-frame.getWidth())/2;
int centerHeight=((int)d.getHeight()-frame.getHeight())/2;
setLocation(centerWidth,centerHeight);

}
public void actionPerformed(ActionEvent e){
Object source=e.getSource();
if(source==closeItem){
System.exit(0);
}

}
}
public class JMenuDemo {

/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
JFrame frame=new JMenuFrame();
frame.pack();
frame.setVisible(true);
}

}