大伙帮我看看空指针异常

[abcd9845]

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class abc extends JFrame{
c a1 = new c();
public abc(){
super("Demo");
this.setSize(300,300);
this.setContentPane(a1);
this.setVisible(true);
}
public static void main(String args[]){
abc z = new abc();
}
}
class c extends JPanel{
JComboBox combo1 = new JComboBox();
JComboBox combo2 = new JComboBox();
JComboBox combo3 = new JComboBox();

public c(){
combo1.addItem("1");
combo1.addItem("2");
this.add(combo1);
this.add(combo2);
this.add(combo3);

combo1.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent evt){

combo2.removeAllItems();
if(combo1.getSelectedItem().equals("1")){


combo2.addItem("1.1");
}

if(combo1.getSelectedItem().equals("2")){

combo2.addItem("2.2");
}
}
});

combo2.addActionListener(new ActionListener(){
public void actionPerformed(ActionEvent evt){

combo3.removeAllItems();
if(combo2.getSelectedItem().equals("1.1")){

combo3.addItem("1.1.1");

}

if(combo2.getSelectedItem().equals("2.2")){

combo3.addItem("2.2.2");
}
}
});
}

}

[zhouqi724]

equals 改为 ==
addItem和removeAllItem冲突

[myhotsun]

package com.borland.samples.welcome;

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class abc
extends JFrame {
c a1 = new c();
public abc() {
super("Demo");
this.setSize(300, 300);
this.setContentPane(a1);
this.setVisible(true);
}

public static void main(String args[]) {
abc z = new abc();
}
}

class c
extends JPanel {
JComboBox combo1 = new JComboBox();
JComboBox combo2 = new JComboBox();
JComboBox combo3 = new JComboBox();

public c() {
combo1.addItem("1");
combo1.addItem("2");
this.add(combo1);
this.add(combo2);
this.add(combo3);

combo1.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {

combo2.removeAllItems();
if ("1".equals(combo1.getSelectedItem())) {
combo2.addItem("1.1");
}

if ("2".equals(combo1.getSelectedItem())) {
combo2.addItem("2.2");
}
}
});

combo2.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent evt) {
combo3.removeAllItems();
if ("1.1".equals(combo2.getSelectedItem())) {
combo3.addItem("1.1.1");
}

if ("2.2".equals(combo2.getSelectedItem())) {
combo3.addItem("2.2.2");
}
}
});
}
}

[myhotsun]

记住这就是常量写在前面的好处，不存在空指针问题

[aaa2003gf]

教训，和和

[zhouqi724]

不可能每次都是常量比较
所以要比较的话如果不确定，用 ==

[tramp73]

顶
myhotsun(科科)和zhouqi724(J2EE不过如此！！！) 说得都有一部分道理,但最好理解透equals()和==的区别

对基本类型的比较equals()比较的是对象的内容(值),==比较的是对象的内存地址

以后再怎么变用这个一套都OK了的

[abcd9845]

能否帮忙给解决以下并且还实现这个功能，谢谢了

[zhouqi724]

equals改为==