java队列(Queue)用法总结[通俗易懂]

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();
}

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);
        }
    }
}

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