java添加关闭叉号_想实现对话框点叉键关闭当前窗口，可添加此功能后，程序编译不出了，是不是少了什么东西。...

控制台的显示

Exception in thread "main" java.lang.NullPointerException

at com.zyn_1.WriteNote.myEvent(WriteNote.java:121)

at com.zyn_1.WriteNote.init(WriteNote.java:49)

at com.zyn_1.WriteNote.(WriteNote.java:19)

at com.zyn_1.WriteNote.main(WriteNote.java:147)

代码如下：

package com.zyn_1;

import java.awt.*;

import java.awt.event.*;

import java.io.*;

import javax.management.RuntimeErrorException;

public class WriteNote {

private Frame f;

private MenuBar mb;

private Menu m1,m2;

private MenuItem mi1,mi2,mi3,mi4,mi5,mi6;

private FileDialog open,save;

private TextArea ta;

private File file;

private Dialog d;

private Label l;

WriteNote(){

init();

}

public void init(){

f = new Frame("记事本");//创建frame窗体

//对窗体进行基本设置

//f.setLayout(new FlowLayout());//设置布局

f.setLocation(300, 70);//位置

f.setSize(600,640);//大小

mb = new MenuBar();

m1 = new Menu("文件");

m2 = new Menu("帮助");

mi1 = new MenuItem("新建");

mi2 = new MenuItem("打开");

mi3 = new MenuItem("保存");

mi4 = new MenuItem("另存为");

mi5 = new MenuItem("退出");

mi6 = new MenuItem("版本");

mb.add(m1);

mb.add(m2);

m1.add(mi1);

m1.add(mi2);

m1.add(mi3);

m1.add(mi4);

m1.add(mi5);

m2.add(mi6);

open = new FileDialog(f,"打开",FileDialog.LOAD);

save = new FileDialog(f,"保存",FileDialog.SAVE);

myEvent();

ta = new TextArea();

d = new Dialog(f,"版本",false);

d.setLocation(400, 250);//位置

d.setSize(300,170);//大小

d.setLayout(new FlowLayout());

l = new Label();

l.setText("制作的一个简单记事本");

d.add(l);

f.add(ta);

f.setMenuBar(mb);

f.setVisible(true);//通过setvisiable(true)显示窗体

}

public void myEvent(){

mi2.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

// TODO Auto-generated method stub

open.setVisible(true);

String dirpath = open.getDirectory();

String FileName = open.getFile();

if(dirpath == null || FileName == null)

return;

ta.setText("");

file =  new File(dirpath,FileName);

try{

BufferedReader bu = new BufferedReader(new FileReader(file));

String line = null;

while((line=bu.readLine())!=null){

ta.append(line+"rn");

}

bu.close();

}

catch(IOException x){

throw new RuntimeErrorException( null, "读取失败");

}

}

});

mi3.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

// TODO Auto-generated method stub

if(file==null){

save.setVisible(true);

String dirpath = save.getDirectory();

String FileName = save.getFile();

if(dirpath == null || FileName == null)

return;

file =  new File(dirpath,FileName);

}

try{

BufferedWriter bu = new BufferedWriter(new FileWriter(file));

String text = ta.getText();

bu.write(text);

bu.close();

}

catch(IOException x){

throw new RuntimeErrorException( null, "读取失败");

}

}

});

mi5.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

// TODO Auto-generated method stub

System.exit(0);

}

});

d.addWindowListener(new WindowAdapter() {

public  void windowClosing(WindowEvent e){

d.setVisible(false);

}

});

f.addWindowListener(new WindowAdapter() {

public  void windowClosing(WindowEvent e){

System.exit(0);

}

});

mi6.addActionListener(new ActionListener() {

@Override

public void actionPerformed(ActionEvent e) {

// TODO Auto-generated method stub

d.setVisible(true);

}

});

}

public static void main(String[] args) {

// TODO Auto-generated method stub

new WriteNote();

}

}