import java.awt.*;
import java.awt.event.*;
import java.util.Random;
public class youxi extends Frame implements ActionListener{
MenuBar menubar;
Menu game,help;
MenuItem begin,exit;
Button buttonget,buttonenter;
Label labelshownum,labelshowresult;
TextField text;
private String u;
youxi(){
setLayout(null);
this.setBounds(0, 0, 240, 320);
MenuBar menubar=new MenuBar();
setMenuBar(menubar);
Menu game=new Menu("游戏");
menubar.add(game);
Menu help=new Menu("帮助");
menubar.add(help);
MenuItem begin=new MenuItem("开始");
game.add(begin);
MenuItem exit=new MenuItem("退出");
begin.addActionListener(this);
exit.addActionListener(this);
game.add(exit);
Button buttonget=new Button("开始");
add(buttonget);
buttonget.setBounds(170, 50, 60, 30);
buttonget.addActionListener(this);
Button buttonenter=new Button("输入");
add(buttonenter);
buttonenter.setBounds(170, 100, 60, 30);
buttonenter.addActionListener(this);
Label labelshownum=new Label();
add(labelshownum);
labelshownum.setBounds(30, 50, 120, 30);
Label labelshowresult=new Label();
add(labelshowresult);
labelshowresult.setBounds(30, 100, 120, 30);
TextField text=new TextField();
add(text);
setVisible(true);
validate();
} /**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
youxi F=new youxi(); } @Override
public void actionPerformed(ActionEvent arg0) {
// TODO Auto-generated method stub
if(arg0.getSource()==exit)
{
System.exit(0);
}
else if(arg0.getSource()==buttonget)
{
Random arandom=new Random();
int[] i = null;
for(int j=0;j<=3;j++)
{
i[j]=arandom.nextInt(10)+1;
}
for(int j=0;j<=3;j++)
{
String u=String.valueOf(i[j]);
}
labelshownum.setText(u);
}
}}
import java.awt.event.*;
import java.util.Random;
public class youxi extends Frame implements ActionListener{
MenuBar menubar;
Menu game,help;
MenuItem begin,exit;
Button buttonget,buttonenter;
Label labelshownum,labelshowresult;
TextField text;
private String u;
youxi(){
setLayout(null);
this.setBounds(0, 0, 240, 320);
MenuBar menubar=new MenuBar();
setMenuBar(menubar);
Menu game=new Menu("游戏");
menubar.add(game);
Menu help=new Menu("帮助");
menubar.add(help);
MenuItem begin=new MenuItem("开始");
game.add(begin);
MenuItem exit=new MenuItem("退出");
begin.addActionListener(this);
exit.addActionListener(this);
game.add(exit);
Button buttonget=new Button("开始");
add(buttonget);
buttonget.setBounds(170, 50, 60, 30);
buttonget.addActionListener(this);
Button buttonenter=new Button("输入");
add(buttonenter);
buttonenter.setBounds(170, 100, 60, 30);
buttonenter.addActionListener(this);
Label labelshownum=new Label();
add(labelshownum);
labelshownum.setBounds(30, 50, 120, 30);
Label labelshowresult=new Label();
add(labelshowresult);
labelshowresult.setBounds(30, 100, 120, 30);
TextField text=new TextField();
add(text);
setVisible(true);
validate();
} /**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
youxi F=new youxi(); } @Override
public void actionPerformed(ActionEvent arg0) {
// TODO Auto-generated method stub
if(arg0.getSource()==exit)
{
System.exit(0);
}
else if(arg0.getSource()==buttonget)
{
Random arandom=new Random();
int[] i = null;
for(int j=0;j<=3;j++)
{
i[j]=arandom.nextInt(10)+1;
}
for(int j=0;j<=3;j++)
{
String u=String.valueOf(i[j]);
}
labelshownum.setText(u);
}
}}
arg0.getSource()==exit
换成
arg0.getSource().equals(exit)
MenuItem begin=new MenuItem("开始");
MenuItem exit=new MenuItem("退出");在方法中重复定义,声明了exit 所以成员变量exit 没有被初始化 所以下面拿到的值是null 换成 begin=new MenuItem("开始");
exit=new MenuItem("退出");就可以了