大家帮忙看一下 为什么编译通过了 运行时却报Exception in thread "main" java.lang.NullPointerException异常???import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
class MyFrame extends Frame
{
private MenuBar mb;
private Menu mt1,mt2,mt3,mt4;
private MenuItem m;
MyFrame(String name){
super(name);
init();
}
public void init()
{
setLayout(new FlowLayout());
setBounds(100,100,400,300);
mb = getMenuBar();
mt1= new Menu("文件");
mt2 = new Menu("编辑");
mt3 = new Menu("帮助");
mt4 = new Menu("点击");
m = new MenuItem("运算");
mt4.add(m);
mb.add(mt1);
mb.add(mt2);
mb.add(mt3);
mb.add(mt4);
this.setMenuBar(mb);
this.setVisible(true);
}
public static void main(String args[])
{
new MyFrame("接收程序");
}
}
import java.awt.event.*;
import javax.swing.*;
class MyFrame extends Frame
{
private MenuBar mb;
private Menu mt1,mt2,mt3,mt4;
private MenuItem m;
MyFrame(String name){
super(name);
init();
}
public void init()
{
setLayout(new FlowLayout());
setBounds(100,100,400,300);
mb = getMenuBar();
mt1= new Menu("文件");
mt2 = new Menu("编辑");
mt3 = new Menu("帮助");
mt4 = new Menu("点击");
m = new MenuItem("运算");
mt4.add(m);
mb.add(mt1);
mb.add(mt2);
mb.add(mt3);
mb.add(mt4);
this.setMenuBar(mb);
this.setVisible(true);
}
public static void main(String args[])
{
new MyFrame("接收程序");
}
}
Gets the menu bar for this frame.Returns:
the menu bar for this frame, or null if this frame doesn't have a menu bar.
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;@SuppressWarnings("serial")
public class MyFrame extends Frame {
private MenuBar mb = new MenuBar();
private Menu mt1, mt2, mt3, mt4;
private MenuItem m; MyFrame(String name) {
super(name);
init();
} public void init() {
setLayout(new FlowLayout());
setBounds(100, 100, 400, 300);
//mb = getMenuBar();
mt1 = new Menu("文件");
mt2 = new Menu("编辑");
mt3 = new Menu("帮助");
mt4 = new Menu("点击");
m = new MenuItem("运算");
mt4.add(m);
mb.add(mt1);
mb.add(mt2);
mb.add(mt3);
mb.add(mt4);
this.setMenuBar(mb);
this.setVisible(true);
} public static void main(String args[]) {
new MyFrame("接收程序"); }
}
情节是详细点