Java Map在遍历过程中删除元素

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Java中的Map如果在遍历过程中要删除元素，除非通过迭代器自己的remove()方法，否则就会导致抛出ConcurrentModificationException异常。JDK文档中是这么描述的：

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 wayexcept 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.

这么做的原因是为了保证迭代器能够尽快感知到Map的“结构性修改“，从而避免不同视图下不一致现象。

下面代码展示了遍历Map时删除元素的正确方式和错误方式。

import java.util.HashMap;import java.util.Iterator;import java.util.Map;import java.util.Set;/** * Created by lh on 15-1-22. */public class TestMapRemove {    public static void main(String[] args){        new TestMapRemove().removeByIterator();//        new TestMapRemove().removeBymap();    }    public void removeByIterator(){//正确的删除方式        HashMap<Integer, String> map = new HashMap<Integer, String>();        map.put(1, "one");        map.put(2, "two");        map.put(3, "three");        System.out.println(map);        Iterator<Map.Entry<Integer, String>> it = map.entrySet().iterator();        while(it.hasNext()){            Map.Entry<Integer, String> entry = it.next();            if(entry.getKey() == 2)                it.remove();//使用迭代器的remove()方法删除元素        }        System.out.println(map);    }    public void removeBymap(){//错误的删除方式        HashMap<Integer, String> map = new HashMap<Integer, String>();        map.put(1, "one");        map.put(2, "two");        map.put(3, "three");        System.out.println(map);        Set<Map.Entry<Integer, String>> entries = map.entrySet();        for(Map.Entry<Integer, String> entry : entries){            if(entry.getKey() == 2){                map.remove(entry.getKey());//ConcurrentModificationException            }        }        System.out.println(map);    }}

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