Queue接口中remove()/poll()、add(e)/offer(e)、element()/peek()的区别 - 每天五分钟搞定Java

JDK源码中Queue接口定义上面的注释如下：

A collection designed for holding elements prior to processing.
 * Besides basic {@link java.util.Collection Collection} operations,
 * queues provide additional insertion, extraction, and inspection
 * operations.  Each of these methods exists in two forms: one throws
 * an exception if the operation fails, the other returns a special
 * value (either {@code null} or {@code false}, depending on the
 * operation).  The latter form of the insert operation is designed
 * specifically for use with capacity-restricted {@code Queue}
 * implementations; in most implementations, insert operations cannot
 * fail.
 *
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 * <caption>Summary of Queue methods</caption>
 *  <tr>
 *    <td></td>
 *    <td ALIGN=CENTER><em>Throws exception</em></td>
 *    <td ALIGN=CENTER><em>Returns special value</em></td>
 *  </tr>
 *  <tr>
 *    <td><b>Insert</b></td>
 *    <td>{@link Queue#add add(e)}</td>
 *    <td>{@link Queue#offer offer(e)}</td>
 *  </tr>
 *  <tr>
 *    <td><b>Remove</b></td>
 *    <td>{@link Queue#remove remove()}</td>
 *    <td>{@link Queue#poll poll()}</td>
 *  </tr>
 *  <tr>
 *    <td><b>Examine</b></td>
 *    <td>{@link Queue#element element()}</td>
 *    <td>{@link Queue#peek peek()}</td>
 *  </tr>
 * </table>
 * ......
 * ......
 * ......

上面这段话从总体上描述了Queue提供的操作都有两种实现方式，一种是操作失败时抛出异常，一种是返回一个特殊值，比如null或者false。这也是remove()/poll()、add(e)/offer(e)、element()/peek()方法的区别所在。

下面的分析基于LinkedList类（实现了Queue接口）的源码

add(e)/offer(e)

一些队列有大小限制，因此如果想在一个满的队列中加入一个新项，多出的项就会被拒绝。这时offer 方法就可以起作用了。它不是对调用 add() 方法抛出一个unchecked 异常，而只是得到由 offer() 返回的 false。

add(E e) 返回true

/**
     * Appends the specified element to the end of this list.
     *
     * <p>This method is equivalent to {@link #addLast}.
     *
     * @param e element to be appended to this list
     * @return {@code true} (as specified by {@link Collection#add})
     */
    public boolean add(E e) {
        linkLast(e);
        return true;
    }

offer(E e) 看方法体可知，是调用了add(E e)方法，可能返回false，也可能返回true

/**
     * Adds the specified element as the tail (last element) of this list.
     *
     * @param e the element to add
     * @return {@code true} (as specified by {@link Queue#offer})
     * @since 1.5
     */
    public boolean offer(E e) {
        return add(e);
    }

remove()/poll()

remove()和poll()都是删除链表头节点，区别如下：
remove() 返回链表的头节点，如果链表为空，则抛出NoSuchElementException异常

/**
     * Retrieves and removes the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E remove() {
        return removeFirst();
    }

poll() 返回链表的头节点，如果链表为空，则返回null

/**
     * Retrieves and removes the head (first element) of this list.
     *
     * @return the head of this list, or {@code null} if this list is empty
     * @since 1.5
     */
    public E poll() {
        final Node<E> f = first;
        return (f == null) ? null : unlinkFirst(f);
    }

element()/peek()

element()和peek()方法都是返回链表头节点，区别如下：
element() 返回链表头节点，如果链表为空，则抛出NoSuchElementException异常

/**
     * Retrieves, but does not remove, the head (first element) of this list.
     *
     * @return the head of this list
     * @throws NoSuchElementException if this list is empty
     * @since 1.5
     */
    public E element() {
        return getFirst();
    }

peek() 返回链表头节点，如果链表为空，则返回null

/**
     * Retrieves, but does not remove, the head (first element) of this list.
     *
     * @return the head of this list, or {@code null} if this list is empty
     * @since 1.5
     */
    public E peek() {
        final Node<E> f = first;
        return (f == null) ? null : f.item;
    }