JDK源码阅读（2）：Queue

Queue

version：1.8

不同于Stack类提供的栈的具体实现，Queue为JDK提供的队列（先进先出）的接口。

/**
 * @see java.util.Collection
 * @see LinkedList
 * @see PriorityQueue
 * @see java.util.concurrent.LinkedBlockingQueue
 * @see java.util.concurrent.BlockingQueue
 * @see java.util.concurrent.ArrayBlockingQueue
 * @see java.util.concurrent.LinkedBlockingQueue
 * @see java.util.concurrent.PriorityBlockingQueue
 * **/
public interface Queue<E> extends Collection<E> 

其定义了六个方法，分为两类，每类三个（进，出，查看队首元素），两类方法的区别主要在于对队列为空（添加失败） 时的处理方法。

第一类：队列为空时返回null

添加失败（非添加元素原因）返回false
队列为空返回null

/**
  * @return {@code true} if the element was added to this queue, else
  *         {@code false}
  * @throws ClassCastException if the class of the specified element
  *         prevents it from being added to this queue
  * @throws NullPointerException if the specified element is null and
  *         this queue does not permit null elements
  * @throws IllegalArgumentException if some property of this element
  *         prevents it from being added to this queue
  */
boolean offer(E e);

//@return the head of this queue, or {@code null} if this queue is empty
E poll();

//@return the head of this queue, or {@code null} if this queue is empty
E peek();

第二类：队列为空抛出异常

若由于容量限制添加失败抛出异常IllegalStateException
队列为空抛出异常NoSuchElementException

/**
  * @return {@code true} (as specified by {@link Collection#add})
  * @throws IllegalStateException if the element cannot be added at this
  *         time due to capacity restrictions
  * @throws ClassCastException if the class of the specified element
  *         prevents it from being added to this queue
  * @throws NullPointerException if the specified element is null and
  *         this queue does not permit null elements
  * @throws IllegalArgumentException if some property of this element
  *         prevents it from being added to this queue
  */
boolean add(E e);

//@return the head of this queue
//@throws NoSuchElementException if this queue is empty
E remove();

//@return the head of this queue
//@throws NoSuchElementException if this queue is empty
E element();