1.佇列的特點

佇列是一種比較特殊的線性結構。它只允許在表的前端（front）進行刪除操作，而在表的後端（rear）進行插入操作。進行插入操作的端稱為隊尾，進行刪除操作的端稱為隊頭。
佇列中最先插入的元素也將最先被刪除，對應的最後插入的元素將最後被刪除。因此佇列又稱為“先進先出”（FIFO—first in first out）的線性表，與棧(FILO-first in last out)剛好相反。

2.java中的佇列

java中的Queue介面就實現了佇列的功能。

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();
}

 

不得不說，JDK裡的程式碼以及註釋看著就是很舒服。各位稍微花點時間看看JDK裡的原始碼以及註釋，相信會有很多收穫。

3.測試Queue介面

JDK中,LinkedList類實現了Queue介面，可以當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);
        }
    }
}

 

最終程式碼執行的結果：

1
2
3
4
----------
poll : 1
----------
2
3
4
ele is: 2
----------
2
3
4
peek : 2
----------
2
3
4

結合第二部分內容，可以有以下結論:
1.儘量使用offer()方法新增元素，使用poll()方法移除元素。dd()和remove()方法在失敗的時候會丟擲異常。
2.peek方法不會刪除元素， Retrieves, but does not remove, the head of this queue,