在java5中新增加了java.util.Queue介面，用以支援佇列的常見操作。

Queue介面與List、Set同一級別，都是繼承了Collection介面。
Queue使用時要儘量避免Collection的add()和remove()方法，而是要使用offer()來加入元素，使用poll()來獲取並移出元素。
新加的方法肯定有好處，不然幹嘛加他，直接用add和remove方法不就好啦。add()和remove()方法在失敗的時候會丟擲異常。
 如果要使用前端而不移出該元素，使用 element()或者peek()方法。
值得注意的是LinkedList類實現了Queue介面，因此我們可以把LinkedList當成Queue來用。
LinkedList實現了Queue介面。

附上使用的示例程式碼，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");//插入一個元素
        queue.offer("2");
        queue.offer("3");
        //列印元素個數
        System.out.println("queue.size()  " + queue.size());//queue.size()  3
        //遍歷列印所有的元素,按照插入的順序列印
        for (String string : queue) {
            System.out.println(string);
        }
        System.out.println("queue.size()  " + queue.size());//queue.size()  3   上面只是簡單迴圈，沒改變佇列

        String getOneFrom1 = queue.element();
        System.out.println("getOneFrom1  " + getOneFrom1);//getOneFrom1  1          因為使用前端而不移出該元素
        System.out.println("queue.size()  " + queue.size());//queue.size()  3       佇列變啦才怪

        String getOneFrom2 = queue.peek();
        System.out.println("getOneFrom2  " + getOneFrom2);//getOneFrom2  1          因為使用前端而不移出該元素
        System.out.println("queue.size()  " + queue.size());//queue.size()  3       佇列變啦才怪

        String getOneFrom3 = queue.poll();
        System.out.println("getOneFrom3  " + getOneFrom3);//getOneFrom3  1          獲取並移出元素
        System.out.println("queue.size()  " + queue.size());//queue.size()  2       佇列變啦
    }
}
 

//摘自Java 1.8 原始碼,註釋摘一半不到.這個介面，就這麼幾個方法，好幸福啊，這麼少。
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多丟了個異常,跟容量相關

    /**
     * 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();//與poll的差別是當佇列是空的時候，這個要報異常，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();//當佇列空的時候異常,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();

}