- 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 中源码解释
/**
* 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();