项目github地址：bitcarmanlee easy-algorithm-interview-and-practice
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1.队列的特点

队列是一种比较特殊的线性结构。它只允许在表的前端（front）进行删除操作，而在表的后端（rear）进行插入操作。进行插入操作的端称为队尾，进行删除操作的端称为队头。
队列中最先插入的元素也将最先被删除，对应的最后插入的元素将最后被删除。因此队列又称为“先进先出”（FIFO—first in first out）的线性表，与栈(FILO-first in last out)刚好相反。

2.java中的队列

java中的Queue接口就实现了队列的功能。

public interface Queue extends Collection {

    /**

     * Inserts the specified element into this queue if it is possible to do so

     * immediately without violating capacity restrictions, returning

     * {@code true} upon success and throwing an {@code IllegalStateException}

     * if no space is currently available.

     *

     * @param e the element to add

     * @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);



    /**

     * Inserts the specified element into this queue if it is possible to do

     * so immediately without violating capacity restrictions.

     * When using a capacity-restricted queue, this method is generally

     * preferable to {@link #add}, which can fail to insert an element only

     * by throwing an exception.

     *

     * @param e the element to add

     * @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);



    /**

     * Retrieves and removes the head of this queue.  This method differs

     * from {@link #poll poll} 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 remove();



    /**

     * Retrieves and removes 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 poll();



    /**

     * 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();

}

不得不说，JDK里的代码以及注释看着就是很舒服。各位稍微花点时间看看JDK里的源码以及注释，相信会有很多收获。

3.测试Queue接口

JDK中,LinkedList类实现了Queue接口，可以当Queue使用。

public class QueueTest {



    @Test

    public void test() {

        Queue queue = new LinkedList<>();

        queue.offer(1);

        queue.offer(2);

        queue.offer(3);

        queue.offer(4);

        for(int e : queue) {

            System.out.println(e);

        }

        System.out.println("----------");

        System.out.println("poll : " + queue.poll());

        System.out.println("----------");



        for(int e : queue) {

            System.out.println(e);

        }

        System.out.println("ele is: " + queue.element());

        System.out.println("----------");



        for(int e : queue) {

            System.out.println(e);

        }



        System.out.println("peek : " + queue.peek());

        System.out.println("----------");



        for(int e : queue) {

            System.out.println(e);

        }

    }

}

最终代码运行的结果：

1

2

3

4

----------

poll : 1

----------

2

3

4

ele is: 2

----------

2

3

4

peek : 2

----------

2

3

4

结合第二部分内容，可以有以下结论:
1.尽量使用offer()方法添加元素，使用poll()方法移除元素。dd()和remove()方法在失败的时候会抛出异常。
2.peek方法不会删除元素， Retrieves, but does not remove, the head of this queue,