Queue

方法

1. boolean offer(E e);
   Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
   returning true upon success and throwing an IllegalStateException if no space is currently available.
2. boolean offer(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 add(E), which can fail to insert an element only by throwing an exception.

3. E remove();
   Retrieves and removes the head of this queue.

This method differs from poll only in that it throws an exception if this queue is empty.

4. E poll();
   Retrieves and removes the head of this queue,

or returns null if this queue is empty.

5. E element();
   **Retrieves, but does not remove, the head of this queue. **

This method differs from peek only in that it throws an exception if this queue is empty.

6. E peek();
   Retrieves, but does not remove, the head of this queue,

or returns null if this queue is empty.

整体描述

A collection designed for holding elements prior to processing.

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Besides basic Collection operations, queues provide additional insertion, extraction, and inspection operations.
**Each of these methods exists in two forms: **

1. one throws an exception if the operation fails,
2. the other returns a special value (either null or false, depending on the operation).

The latter form of the insert operation is designed specifically for use with capacity-restricted Queue implementations; in most implementations, insert operations cannot fail.

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Throws exception
Returns special value
Insert
add(e)
offer(e)
Remove
remove()
poll()
Examine
element()
peek()

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fifo vs lifo

1. fifo
   Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner.
2. priority
   Among the exceptions are priority queues, which order elements according to a supplied comparator, or the elements' natural ordering,
3. lifo
   and LIFO queues (or stacks) which order the elements LIFO (last-in-first-out). Whatever the ordering used, the head of the queue is that element which would be removed by a call to remove() or poll().

In a FIFO queue, all new elements are inserted at the tail of the queue.
Other kinds of queues may use different placement rules. Every Queue implementation must specify its ordering properties.

offer vs add

1. offer
   The offer method inserts an element if possible, otherwise returning false.
2. add
   This differs from the Collection.add method, which can fail to add an element only by throwing an unchecked exception.
3. vs
   The offer method is designed for use** when failure is a normal, rather than exceptional occurrence,

remove poll
The remove() and poll() methods remove and return the head of the queue.

Exactly which element is removed from the queue is a function of the queue's ordering policy, which differs from implementation to implementation.

1. remove
   The remove() and poll() methods differ only in their behavior when the queue is empty: the remove() method throws an exception,
   2.poll
   while the poll() method returns null.
   The element() and peek() methods return, but do not remove, the head of the queue.

不定义阻塞队列的方法
The Queue interface does not define the blocking queue methods, which are common in concurrent programming.
These methods, which wait for elements to appear or for space to become available, are defined in the BlockingQueue interface, which extends this interface.

null
Queue implementations generally do not allow insertion of null elements,

1. although some implementations, such as LinkedList, do not prohibit insertion of null.
2. Even in the implementations that permit it, null should not be inserted into a Queue, as null is also used as a special return value by the poll method to indicate that the queue contains no elements.