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在 Queue 中 element() 和 peek()有什么区别？
Queue 中 element() 和 peek()都是用来返回队列的头元素，不删除。
在队列元素为空的情况下，element() 方法会抛出NoSuchElementException异常，peek() 方法只会返回 null。
JDK1.8
/**
* Retrieves, but does not remove, the head of this queue. This method
* differs from {@link #peek peek} only in that it throws an exception
* if this queue is empty.
*
* @return the head of this queue
* @throws NoSuchElementException if this queue is empty
*/
E element();
/**
* Retrieves, but does not remove, the head of this queue,
* or returns {@code null} if this queue is empty.
*
* @return the head of this queue, or {@code null} if this queue is empty
*/
E peek();