异常：Exception in thread "main" java.util.ConcurrentModificationException

错误代码：

Map<Integer,Integer> map = new HashMap<>();
map.put(1,1);
map.put(2,2);
map.put(3,3);
map.put(4,4);
for (Map.Entry<Integer,Integer> entry : map.entrySet()) {
int key = entry.getKey();
int value = entry.getValue();
map.remove(key);
}