程序如下：
package com.zyn_1;
import java.awt.*;
import java.awt.event.*;
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;
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
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);
ta = new TextArea();
f.add(ta);
myEvent();
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);
}
});
mi5.addActionListener(new ActionListener() {
@Override
public void actionPerformed(ActionEvent e) {
// TODO Auto-generated method stub
System.exit(0);
}
});
f.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e){
System.exit(0);
}
});
}
public static void main(String[] args) {
// TODO Auto-generated method stub
new WriteNote();
}
}
控制台显示内容：
Exception in thread "main" java.lang.NullPointerException
at com.zyn_1.WriteNote.init(WriteNote.java:32)
at com.zyn_1.WriteNote.<init>(WriteNote.java:15)
at com.zyn_1.WriteNote.main(WriteNote.java:78)
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