这里是ConcurrentModificationException()的解释:
http://docs.oracle.com/javase/1.5.0/docs/api/java/util/ConcurrentModificationException.html
我贴一下吧
This exception may be thrown by methods that have detected concurrent modification of an object when such modification is not permissible.
For example, it is not generally permissible for one thread to modify a Collection while another thread is iterating over it. In general, the results of the iteration are undefined under these circumstances. Some Iterator implementations (including those of all the general purpose collection implementations provided by the JRE) may choose to throw this exception if this behavior is detected. Iterators that do this are known as fail-fast iterators, as they fail quickly and cleanly, rather that risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that this exception does not always indicate that an object has been concurrently modified by a different thread. If a single thread issues a sequence of method invocations that violates the contract of an object, the object may throw this exception. For example, if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception.
Note that fail-fast behavior cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast operations throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: ConcurrentModificationException should be used only to detect bugs.
说的是为了防止迭代器遍历的时候集合被修改,机制原理:
执行next方法的时候会首先执行 checkForComodification
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
expectedModCount其实就是集合的大小,而modCount 初始的时候也是集合的大小,但是每修改一次集合,modCount 就会加一,再执行next的时候就会抛出ConcurrentModificationException异常了。
值得注意的是删除集合的倒数第二个元素 会导致hasnext方法返回false,就不会执行next方法,然后就不会报错了。这也是jdk考虑不周全的地方吧。
public boolean hasNext() {
return cursor != size();
}
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