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.
译文：这个类的“集合视图方法”返回的迭代器是快速失败的:如果在创建迭代器之后的任何时候结构上都被修改了，那么除了通过迭代器自己的删除方法外，迭代器将抛出ConcurrentModificationException。因此，在面对并发修改时，迭代器会快速而干净地失败，而不会在将来的某个不确定的时间冒险使用任意的、不确定的行为。

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

public class HashTest {

    public static void main(String[] args) {

        HashMap<Integer, Integer> count = new HashMap<Integer, Integer>();

        count.put(1, 11);

        count.put(2, 22);

        count.put(3, 33);

        //错误的 会抛出异常 ----ConcurrentModificationException

        for (Integer i : count.keySet()) {

            if(i == 2){

                count.remove(i);

            }

            System.out.println(i);

        }

        //正确的

        Iterator<Integer> it = count.keySet().iterator();

        while(it.hasNext()) {

            Integer key = it.next();

            if(key == 3){

                count.put(key, 44);

            }

        }

        for (Integer value : count.values()) {

            System.out.println(value);

        }

    }

}