java.util.ConcurrentModificationException异常 解决方法

原文：http://blog.sina.com.cn/s/blog_48dc562301000d2b.html

今天写一个程序的时候遇到了java.util.ConcurrentModificationException异常，半天不知道是什么问题，这里举个简单的例子重现一下异常是如何出现的：

import java.util.ArrayList;
import java.util.Iterator;

class User {
    private String userName, password;
    private int ID;

    User(int p_ID, String p_userName, String p_password) {
        ID = p_ID;
        userName = p_userName;
        password = p_password;
     }
}

public class Test {
    static ArrayList al = new ArrayList();

    public static void main(String[] args) {
        add();
        clear();
     }

    public static void add() {
        for (int i = 0; i < 5; i++) {
            al.add(new User(i, "userName", "password"));
         }
     }

    public static void clear() {
        Iterator it = ((ArrayList) al).iterator();
        while (it.hasNext()) {
            User usr = (User) (it.next());
            int index = al.indexOf(usr);
            al.remove(index);
            // it.remove();
        }
        System.out.println("Cleard");
     }
}