• 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();