集合的ConcurrentModification异常

引用http://www.blogjava.net/sitinspring/archive/2007/12/03/165006.html

在以下四种遍历过程中,前两种会抛出ConcurrentModificationException,而后两种方法是正确的.

Department类:
package com.sitinspring;

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

public class Department {
    private String name;

    private List<Member> memberSheet;

    public Department(String name) {
        this.name = name;
    }

    public void addMemer(Member member) {
        if (memberSheet == null) {
            memberSheet = new ArrayList<Member>();
        }

        memberSheet.add(member);
    }

    public void printMemberSheet() {
        System.out.println("----部门" + name + "人员名单---");

        for (Member member : memberSheet) {
            System.out.println(member);
        }
    }

    /**
     * 里面的四个清除过程请分别独立执行
     *
     */
    public void removeYoungerFromMemberSheet() {   
        // 遍历一:这个处理会抛出java.util.ConcurrentModificationException
        for (Member member : memberSheet) {
            if (member.getAge() < 30) {
                memberSheet.remove(member);
            }
        }
       
        // 遍历二:这个处理也会抛出java.util.ConcurrentModificationException
        for (Iterator it = memberSheet.iterator(); it.hasNext();) {
            Member member = (Member) it.next();

            if (member.getAge() < 30) {
                memberSheet.remove(member);
            }
        }
       
        // 遍历三:这个处理调用Iterator 本身的方法 remove(),会正常执行
        for (Iterator it = memberSheet.iterator(); it.hasNext();) {
            Member member = (Member) it.next();

            if (member.getAge() < 30) {
                it.remove();
            }
        }
       
        // 遍历四:这个处理不依赖Iterator,也会正常执行
        for (int i=0;i<memberSheet.size();i++) {
            Member member = memberSheet.get(i);

            if (member.getAge() < 30) {
                memberSheet.remove(member);
            }
        }
    }

    public String toString() {
        return name;
    }

    public String getName() {
        return name;
    }

    public static void main(String[] args) {
        Department resarchDept = new Department("研发部门");
        resarchDept.addMemer(new Member("张三", 38));
        resarchDept.addMemer(new Member("李四", 24));
        resarchDept.addMemer(new Member("王五", 30));
        resarchDept.addMemer(new Member("钱七", 22));
        resarchDept.addMemer(new Member("孙八", 39));
        resarchDept.addMemer(new Member("周九", 30));

        resarchDept.removeYoungerFromMemberSheet();
        resarchDept.printMemberSheet();
    }
}

Member类:
package com.sitinspring;

public class Member{
    private String name;
    private int age;
    private Department department;
   
    public Member(String name,int age){
        this.name=name;
        this.age=age;
    }
   
    public String toString(){
        StringBuffer sb=new StringBuffer();
        sb.append("员工名="+name);
        sb.append(" 年龄="+age);
       
        if(department!=null){
            sb.append(" 所属部门="+department);
        }
       
        return sb.toString();
    }
   
    public void changeNewDepartment(Department newDepartment) {
        department.removeMemer(name);
        newDepartment.addMemer(this);
    }
   
    public String getName() {
        return name;
    }

    public void setDepartment(Department department) {
        this.department = department;
    }
}

为什么会发生这样的结果呢?这是因为

"当使用 fail-fast iterator 对 Collection 或 Map 进行迭代操作过程中尝试直接修改 Collection / Map 的内容时，即使是在单线程下运行， java.util.ConcurrentModificationException 异常也将被抛出。
Iterator 是工作在一个独立的线程中，并且拥有一个 mutex 锁。 Iterator 被创建之后会建立一个指向原来对象的单链索引表，当原来的对象数量发生变化时，这个索引表的内容不会同步改变，所以当索引指针往后移动的时候就找不到要迭代的对象，所以按照 fail-fast 原则 Iterator 会马上抛出 java.util.ConcurrentModificationException 异常。
所以 Iterator 在工作的时候是不允许被迭代的对象被改变的。但你可以使用 Iterator 本身的方法 remove() 来删除对象， Iterator.remove() 方法会在删除当前迭代对象的同时维护索引的一致性。"

上述这段资料来自http://hi.baidu.com/xjenator/blog/item/23b235a89041d4b0ca130c16.html.

java.util包中很多迭代器都是所谓的fail-fast迭代器.这些迭代器如果发现集合被修改,而且不是通过迭代器本身,那么抛出一个异常进行清除-ConcurrentModificationException-从而避免不安全行为的发生.

因此,第三种采用it.remove();不会出现任何异常,而第四不依赖于Iterator而依赖于索引当然更不会出现异常.