跪求，解决方案

Question

自己编了一个程序，程序的功能是在三个JComboBox 对象里面，选择对应的项目，但是总是不能达到我的目的；就是依次选择
县，乡，村，程序的代码，是；
import javax.swing.*; 
import java.awt.event.*;
import java.sql.*; 
public class Example12_2 {
public static void main(String args[]){
 try { Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");  
         }
         catch(ClassNotFoundException e1){
               System.out.println(""+e1);
         }
           new Mysql();
}
}
class Mysql extends JFrame implements ItemListener {
 JComboBox box1;
 JComboBox box2;
JComboBox box3;
Connection con;
 Statement sql;
 ResultSet rs;
   Connection con1;
 Statement sql1;
 ResultSet rs1;
 
Mysql(){
     box1=new  JComboBox();
     box2=new  JComboBox();
     box3=new  JComboBox();
    setBounds(100,100,380,200);
        setVisible(true);  
        setLayout(null);
        box1.addItem("请选择县");
        box1.addItem("垫江县");
        box1.addItem("梁平县");
        box2.addItem("请选择乡");
        box3.addItem("请选择村");
        add(box1);
        add(box2);
        add(box3);
        box1.setBounds(20, 50, 100, 50);
        box2.setBounds(140, 50, 100, 50);
        box3.setBounds(260, 50, 100, 50);
        validate();
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        box1.addItemListener(this);
        box2.addItemListener(this);
         
         
 }
 public void itemStateChanged(ItemEvent e){
          if(box1==e.getSource()){
          box2.removeAllItems();
                box3.removeAllItems();
              box2.addItem("请选择乡");
            box3.addItem("请选择村");
           
            String  str=box1.getSelectedItem().toString();
               
            String  str1="SELECT * FROM "+str;
                
              
               try{ con=DriverManager.getConnection("jdbc:odbc:eleven","","");
               sql= con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_READ_ONLY);
               rs=sql.executeQuery(str1);
                    
                       while(rs.next()){
                       box2.addItem( rs.getString(1).trim());
                      
                       }
                      
                      con.close();
                       getContentPane().removeAll();
                       add(box1);
                       add(box2);
                       add(box3);
                       validate();
                      
               }
              
               catch(SQLException e2){
                          System.out.println(e2);
              
               }
           
           }
               
          
        
          else if(box2==e.getSource()){
         
               box3.removeAllItems();
               box3.addItem("请选择村");
              String  str=box2.getSelectedItem().toString();
              
              String  str1="SELECT * FROM "+str;
               
             
              try{ con=DriverManager.getConnection("jdbc:odbc:eleven","","");
              sql= con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_READ_ONLY);
              rs=sql.executeQuery(str1);
                      while(rs.next()){
                      box3.addItem( rs.getString(1));
                     
                      }
                   
                      con.close();
                      
              }
              catch(SQLException e2){
                         System.out.println(e2);
                   }
             
               }
        
}
}
相应的数据库是，里面的表是；表名；垫江县；晓兴乡，周家乡
表名；垫江县；属性列；县，晓兴乡，周家乡
表名；梁平县；属性列；县，大岩乡，阴平乡表名；晓兴乡；属性列；乡，努力村，幸福村 
表名；周家乡；属性列；乡，建国村，努力村表名；大岩乡；属性列；乡，和谐村，美好村
表名；阴平乡；属性列；乡，我们村，他们村

Answer 1

if(box1==e.getSource()){ 
...
 getContentPane().removeAll();
 add(box1);
 add(box2);
 add(box3); 为什么要removeAll()再add()?

Answer 2

import javax.swing.*;
import java.awt.event.*;
import java.sql.*;
public class Example12_2 {
public static void main(String args[]){
try { Class.forName("sun.jdbc.odbc.JdbcOdbcDriver"); 
        }
        catch(ClassNotFoundException e1){
              System.out.println(""+e1);
        }
          new Mysql();
}
}
class Mysql extends JFrame implements ItemListener {
JComboBox box1;
JComboBox box2;
JComboBox box3;
Connection con;
Statement sql;
ResultSet rs;
Connection con1;
Statement sql1;
ResultSet rs1;Mysql(){
  box1=new  JComboBox();
  box2=new  JComboBox();
  box3=new  JComboBox();
  setBounds(100,100,380,200);
        setVisible(true); 
        setLayout(null);
        box1.addItem("请选择县");
        box1.addItem("垫江县");
        box1.addItem("梁平县");
        box2.addItem("请选择乡");
        box3.addItem("请选择村");
        add(box1);
        add(box2);
        add(box3);
        box1.setBounds(20, 50, 100, 50);
        box2.setBounds(140, 50, 100, 50);
        box3.setBounds(260, 50, 100, 50);
        validate();
        setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        box1.addItemListener(this);
        box2.addItemListener(this);
       
       
}
public void itemStateChanged(ItemEvent e){
        if(box1==e.getSource()){
        box2.removeAllItems();
              box3.removeAllItems();
            box2.addItem("请选择乡");
          box3.addItem("请选择村");
         
          String  str=box1.getSelectedItem().toString();
             
          String  str1="SELECT * FROM "+str;
             
             
              try{ con=DriverManager.getConnection("jdbc:odbc:eleven","","");
              sql= con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_READ_ONLY);
              rs=sql.executeQuery(str1);
                   
                      while(rs.next()){
                      box2.addItem( rs.getString(1).trim());
                     
                      }
                     
                      con.close();
                      getContentPane().removeAll();
                      add(box1);
                      add(box2);
                      add(box3);
                      validate();
                     
              }
             
              catch(SQLException e2){
                          System.out.println(e2);
             
              }
       
        }
           
       
       
        else if(box2==e.getSource()){
       
              box3.removeAllItems();
              box3.addItem("请选择村");
            String  str=box2.getSelectedItem().toString();
             
            String  str1="SELECT * FROM "+str;
           
           
            try{ con=DriverManager.getConnection("jdbc:odbc:eleven","","");
            sql= con.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_READ_ONLY);
            rs=sql.executeQuery(str1);
                      while(rs.next()){
                    box3.addItem( rs.getString(1));
                   
                      }
                 
                      con.close();
                     
            }
            catch(SQLException e2){
                        System.out.println(e2);
                  }
           
            }
       
}
}