在java5中新增加了java.util.Queue接口，用以支持队列的常见操作。

Queue接口与List、Set同一级别，都是继承了Collection接口。
Queue使用时要尽量避免Collection的add()和remove()方法，而是要使用offer()来加入元素，使用poll()来获取并移出元素。
新加的方法肯定有好处，不然干嘛加他，直接用add和remove方法不就好啦。add()和remove()方法在失败的时候会抛出异常。
 如果要使用前端而不移出该元素，使用 element()或者peek()方法。
值得注意的是LinkedList类实现了Queue接口，因此我们可以把LinkedList当成Queue来用。
LinkedList实现了Queue接口。

附上使用的示例代码，如下：

import java.util.LinkedList; 
import java.util.Queue; 
 
class Hello { 
    public static void main(String[] a) { 
        Queue<String> queue = new LinkedList<>(); 
        queue.offer("1");//插入一个元素 
        queue.offer("2"); 
        queue.offer("3"); 
        //打印元素个数 
        System.out.println("queue.size()  " + queue.size());//queue.size()  3 
        //遍历打印所有的元素,按照插入的顺序打印 
        for (String string : queue) { 
            System.out.println(string); 
        } 
        System.out.println("queue.size()  " + queue.size());//queue.size()  3   上面只是简单循环，没改变队列 
 
        String getOneFrom1 = queue.element(); 
        System.out.println("getOneFrom1  " + getOneFrom1);//getOneFrom1  1          因为使用前端而不移出该元素 
        System.out.println("queue.size()  " + queue.size());//queue.size()  3       队列变啦才怪 
 
        String getOneFrom2 = queue.peek(); 
        System.out.println("getOneFrom2  " + getOneFrom2);//getOneFrom2  1          因为使用前端而不移出该元素 
        System.out.println("queue.size()  " + queue.size());//queue.size()  3       队列变啦才怪 
 
        String getOneFrom3 = queue.poll(); 
        System.out.println("getOneFrom3  " + getOneFrom3);//getOneFrom3  1          获取并移出元素 
        System.out.println("queue.size()  " + queue.size());//queue.size()  2       队列变啦 
    } 
}

//摘自Java 1.8 源码,注释摘一半不到.这个接口，就这么几个方法，好幸福啊，这么少。 
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. 
     * ...... 
     */ 
    boolean add(E e);//比offer多丢了个异常,跟容量相关 
 
    /** 
     * 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. 
     * ...... 
     */ 
    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. 
     * ...... 
     */ 
    E remove();//与poll的差别是当队列是空的时候，这个要报异常，下面返回null 
 
    /** 
     * Retrieves and removes the head of this queue, 
     * or returns {@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. 
     * ...... 
     */ 
    E element();//当队列空的时候异常,下面的只是返回null 
 
    /** 
     * Retrieves, but does not remove, the head of this queue, 
     * or returns {@code null} if this queue is empty. 
     * ...... 
     */ 
    E peek(); 
 
}