Queue remove/poll, add/offer, element/peek整理

offer／add

• 相关add方法

对于AbstractQueue等实现为：
public boolean add(E e) {
        if (offer(e))
            return true;
        else
            throw new IllegalStateException("Queue full");
}
而对于大多数实现为：或者直接调用offer操作
public boolean add(E e) {
        linkLast(e);
        return true;
}
void linkLast(E e) {
        final Node<E> l = last;
        final Node<E> newNode = new Node<>(l, e, null);
        last = newNode;
        if (l == null)
            first = newNode;
        else
            l.next = newNode;
        size++;
        modCount++;
 }

image.gif

官方对于add方法的描述是：

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
即：如果容量允许会立即插入指定元素到队列并返回success，如果没有足够的容量，会抛出异常

image.gif

• 相关offer方法：

public boolean offer(E e) {
        return add(e);
}

image.gif

官方对于offer方法的描述是：

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.
  即：在没有容量限制尽可能快的插入指定元素到队列，当使用一个容量限制的队列，这个方法一般是优于add方法，后者(即add)在插入元素失败会引发异常

image.gif

• 总结：

对于有队列大小限制，如果队列在满队列的情况下，这时候add方法会执行失败，抛出异常，这时候如果offer操作，则只会返回false

remove/poll

• 相关poll

官方解读：

Retrieves and removes the head of this queue, or returns {@code null} if this queue is empty.

image.gif

• 相关remove

官方解读：

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.

image.gif

• 总结：

这两个方法对比很明显，都是查询并移除队列的头元素，不同的是如果队列为empty，remove会抛出异常，poll则会返回null

element/peek

• element：

Retrieves, but does not remove, the head of this queue, or returns {@code null} if this queue is empty.

image.gif

• peek：

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.

image.gif

• 总结：

它俩功能是查询队列头部元素但不移除元素，与之前的remove/poll一样，element/peek也是诸如此类的区别，在队列为empty时，element会抛出一个异常，而peek会返回null.

对于queue的几个容易混淆的方法整理就到这