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 map = new HashMap();

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> iter = map.entrySet().iterator();

while (iter.hasNext()) {

Map.Entry 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 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