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 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(“------- 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