Iterator中的next()

DBExchangeMoney类：
  1 package com.ch.test15;
  2 
  3 import java.sql.DriverManager;
  4 import java.sql.ResultSet;
  5 import java.sql.SQLException;
  6 import java.sql.Statement;
  7 import java.sql.Timestamp;
  8 import java.util.ArrayList;
  9 import java.util.Date;
 10 import java.util.Iterator;
 11 
 12 import com.ch.test02.Person;
 13 
 14 /**
 15  * 交换A,B,C的金额
 16  * @author Administrator
 17  *
 18  */
 19 public class DBExchangeMoney {
 20     public static void main(String[] args) throws ClassNotFoundException, SQLException {
 21         //定义数据库驱动器
 22         String DRIVER = "com.mysql.jdbc.Driver";
 23         //加载数据库驱动器
 24         Class.forName(DRIVER);
 25         //定义数据库的链接
 26         String URL = "jdbc:mysql://192.168.1.20:3306/cuihao?useUnicode=true&characterEncoding=UTF-8";
 27         String Users = "root";
 28         String Passw = "root";
 29         java.sql.Connection con = null;
 30         try {
 31             con = DriverManager.getConnection(URL, Users, Passw);
 32             con.setAutoCommit(false);
 33         } catch (SQLException e) {
 34             e.printStackTrace();
 35         }
 36         //PreparedStatement prepStmt = null;
 37         Statement st = null;
 38         ResultSet rs = null;
 39         int tem = 0;
 40         ArrayList<MoneyDefine> list = new ArrayList<MoneyDefine>();
 41         try {
 42             //执行sql查询
 43             String selectStatement = "select * from Moneys";
 44             st = con.createStatement();
 45             rs = st.executeQuery(selectStatement);
 46             while (rs.next()) {
 47                 String id = rs.getString("id");
 48                 String name = rs.getString("pname");
 49                 int moneys = rs.getInt("prices");
 50                 String dates = rs.getString("dates");
 51                 MoneyDefine md = new MoneyDefine(id,name,moneys,dates);
 52                 list.add(md);
 53             }
 54             for(Iterator<MoneyDefine> iterator = list.iterator();iterator.hasNext();){
 55                 //错误的，next()在list内，跳到下一条的下一个位置                
 56                 //System.out.println(iterator.next().getId() +iterator.next().getName() 
                    //                   +iterator.next().getMoneys() +iterator.next().getDates());
 57                 //用对象p接收
 58                 MoneyDefine p = iterator.next();
 59                 System.out.println("ID:" +p.getId() +" 姓名：" +p.getName()
 60                         +" 金额：" +p.getMoneys() +" 时间：" +p.getDates());
 61 //                System.out.println(p.getMoneys());
 62 //                System.out.println(p.getDates());
 63             }
 64         
 65             System.out.println("--------------------------");
 66             //two给one 1000; one给two 1000;
 67             //更新
 68             String exchangeMoney = "UPDATE Moneys SET prices=prices+1000 WHERE id=1";
 69             String exchangeMoney1 = "UPDATE Moneys SET prices=prices-1000 WHERE id=2";
 70             st = con.createStatement();
 71             tem = st.executeUpdate(exchangeMoney);
 72             tem = st.executeUpdate(exchangeMoney1);
 73             System.out.println("执行后:" +tem);
 74             //查询更新后的内容
 75             String selectStatement1 = "select * from Moneys";
 76             st = (Statement) con.createStatement();
 77             rs = st.executeQuery(selectStatement1);
 78             while (rs.next()) {
 79                 String id = rs.getString("id");
 80                 String name = rs.getString("pname");
 81                 String moneys = rs.getString("prices");
 82                 Date dates = rs.getTimestamp("dates");
 83                 System.out.print("ID:" +id);
 84                 System.out.print(" 姓名:" +name);
 85                 System.out.print(" 金额:" +moneys);
 86                 System.out.print(" 时间：" +dates);
 87                 System.out.println();
 88             }
 89             System.out.println("--------------------");
 90         } catch (SQLException e) {
 91             con.rollback();
 92             e.printStackTrace();
 93         }
 94         //关闭
 95         con.commit();
 96         rs.close();
 97         st.close();
 98         con.close();
 99     }
100 }

MoneyDefine类：
 1 package com.ch.test15;
 2 
 3 import java.sql.Timestamp;
 4 import java.util.Date;
 5 
 6 public class MoneyDefine {
 7 
 8     public String id;
 9     public String name;
10     public int moneys;
11     public String dates;
12     
13     public MoneyDefine(String id, String name, int moneys, String dates){
14         this.id = id;
15         this.name = name;
16         this.moneys = moneys;
17         this.dates = dates;
18     }
19     public String getId() {
20         return id;
21     }
22     public void setId(String id) {
23         this.id = id;
24     }
25     public String getName() {
26         return name;
27     }
28     public void setName(String name) {
29         this.name = name;
30     }
31     public float getMoneys() {
32         return moneys;
33     }
34     public void setMoneys(int moneys) {
35         this.moneys = moneys;
36     }
37     public String getDates() {
38         return dates;
39     }
40     public void setDates(String dates) {
41         this.dates = dates;
42     }
43     
44     
45 }

DBExchangeMoney类中，将从数据库取出的元素放入ArrayList中，然后再从list里取出。
这时遇到问题，当用迭代取元素时

System.out.println(iterator.next().getId() +iterator.next().getName() +iterator.next().getMoneys() +iterator.next().getDates());

执行出行错误。

修改为：

1 MoneyDefine p = iterator.next();
2 System.out.println("ID:" +p.getId() +" 姓名：" +p.getName()+" 金额：" +p.getMoneys() +" 时间：" +p.getDates());

View Code

正确输出。

如果每次都调用iterator.next()，则第一次调用iterate.next()，指向List的第一行的getId()，即id=1，第二次调用iterate.next()，就会指向第二行的getName()，即name=two。第三次调用，就会报出异常。所以无法正常输出！

id	name	prices	dates
1	one	0	2015-04-15 11:10:34
2	two	1000	2015-04-15 11:10:40

当iterator.next()应该赋值给一个对象p，然后调用对象p来取值，这样p每次会从每行的首个元素开始迭代。

JDK API 1.6，关于Iterator中next()的说明：

next()

返回迭代的下一个元素。

返回：

迭代的下一个元素。

抛出：

NoSuchElementException - 没有元素可以迭代。