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

推广

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

来源：oschina

链接：https://my.oschina.net/u/3136014/blog/1622967

标签

collectionview

java

Entry

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