是使用集合的iterator后,再改变就可能抛出这个异常
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.
iterators是fail-fast的,如果这个集合(存在特定的某些集合,比如HashMap)在iterator创建之后在结构上又改变了(通
过任何方式除了iterator本身的remove方法),iterator就会抛ConcurrentModificationException。因
此,在使用过程中又面临同时被修改,iterator就会以又快速又干净地fail来应对,而不是在将来某个不确定的时间冒面临随机又不确定的表现之风险