Java Map在遍历过程中删除元素

map遍历判断筛选删除时

• 如果对map使用put、remove或clear方法（例如map.remove直接删除），那么迭代器就不再合法（并且在其后使用该迭代器将会有ConcurrentModificationException异常被抛出）.
• 当Iterator.remove方法导致map发生变化时，他会更新cursor来同步这一变化。

参见jdk文档描述：

The iterators returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

结论: 应该使用迭代删除

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针对其他list等集合，遍历过程中的删除操作，也需要使用迭代删除

测试demo

private static Map<Integer, String> map=new HashMap<Integer,String>();

public static void iterTest(){
    map.put(1,"one");
    map.put(2,"two");
    map.put(3,"three");
    map.put(4,"four");
    map.put(5,"five");
    map.put(6,"six");
    map.put(7,"seven");
    map.put(8,"eight");
    map.put(5,"five");
    map.put(9,"nine");
    map.put(10,"ten");

    Iterator<Map.Entry<Integer, String>> iter = map.entrySet().iterator();
    while(iter.hasNext()){
        Map.Entry<Integer, String> entry=iter.next();
        int key=entry.getKey();
        if(key%2==1){
            System.out.println("delete this: "+key+" = "+key);
            //map.put(key, "奇数");   //ConcurrentModificationException
            //map.remove(key);      //ConcurrentModificationException
            iter.remove();        //OK
        }
    }
    //遍历当前的map；这种新的for循环无法修改map内容，因为不通过迭代器。
    System.out.println("-------\n\t最终的map的元素遍历：");
    for(Map.Entry<Integer, String> entry:map.entrySet()){
        int k=entry.getKey();
        String v=entry.getValue();
        System.out.println(k+" = "+v);
    }
}

在main方法中运行 iterTest() ,输出结果为：

-------
	最终的map的元素遍历：
2 = two
4 = four
6 = six
8 = eight
10 = ten

若将

iter.remove(); 

替换成

map.put(key, "奇数");
或者 map.remove(key);

则会报出 ConcurrentModificationException 异常