Java之Queue接口中add()/offer()、remove()/poll()、element()/peek()的區別

隊列的使用在日常開發中，特別常見，但是對於隊列接口中的一些方法可能使用起來有些疑惑，本文簡單記錄一下關於Queue接口中幾種類似方法的區別。

• add() 和 offer()
  • add() : 添加元素，如果添加成功則返回true，如果隊列是滿的，則拋出異常
  • offer() : 添加元素，如果添加成功則返回true，如果隊列是滿的，則返回false
    區別：對於一些有容量限制的隊列，當隊列滿的時候，用add()方法添加元素，則會拋出異常，用offer()添加元素，則返回false
• remove() 和 poll()
  • remove() : 移除隊列頭的元素並且返回，如果隊列爲空則拋出異常
  • poll() : 移除隊列頭的元素並且返回，如果隊列爲空則返回null
    區別：在移除隊列頭元素時，當隊列爲空的時候，用remove()方法會拋出異常，用poll()方法則會返回null
• element() 和 peek()
  • element() ：返回隊列頭元素但不移除，如果隊列爲空，則拋出異常
• peek() ：返回隊列頭元素但不移除，如果隊列爲空，則返回null
  區別 ：在取出隊列頭元素時，如果隊列爲空，用element()方法則會拋出異常，用peek()方法則會返回null

附上源碼以及中文註釋：

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.
     * 插入一個具體的元素到隊列中如果沒有超過容量限制並且返回true。
     * 如果沒有隊列已滿則拋出IllegalStateException
     *
     * @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
     *         如果插入的對象不對，則會拋出類型轉換ClassCastException
     * @throws NullPointerException if the specified element is null and
     *         this queue does not permit null elements
     *         如果插入null，則會拋出空指針異常
     * @throws IllegalArgumentException if some property of this element
     *         prevents it from being added to this queue
     * 添加元素，如果添加成功則返回true，如果隊列是滿的，則拋出異常
     */
    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.
     * 插入一個具體的元素到隊列中如果沒有超過容量限制並且返回true。
     * 如果沒有隊列已滿則返回false。
     * 當使用一個有容量限制的隊列時，建議使用該方法offer(),因爲使用add()方法當隊列滿時會
     * 直接拋出異常，則offer()方法則返回false
     *
     * @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
     * 添加元素，如果添加成功則返回true，如果隊列是滿的，則返回false
     */
    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
     * 移除隊列頭的元素並且返回，如果隊列爲空則拋出異常
     */
    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
     * 移除隊列頭的元素並且返回，如果隊列爲空則返回null
     */
    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
     * 返回隊列頭元素但不移除，如果隊列爲空，則拋出異常
     */
    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
     * 返回隊列頭元素但不移除，如果隊列爲空，則返回null
     */
    E peek();
}