大伙帮我看看空指针异常

Question

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");
}
}
});
}

}

Answer 1

equals 改为 ==
addItem和removeAllItem冲突

Answer 2

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");
        }
      }
    });
  }
}

Answer 3

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