Java Map在遍历过程中删除元素

逃离我推掉我的手 2024-04-17 16:08 61阅读 0赞

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 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.

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);
        }
    }