项目github地址：bitcarmanlee easy-algorithm-interview-and-practice
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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。
java中的Queue接口就实现了队列的功能。
public interface Queue<E> extends Collection<E> { /** * 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<Integer> 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：
1234----------poll : 1----------234ele is: 2----------234peek : 2----------234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,