/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package java.util;
/**
* 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 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.
*
*
*
*
* |
* Throws exception |
* Returns special value |
*
*
* | Insert |
* {@link #add add(e)} |
* {@link #offer offer(e)} |
*
*
* | Remove |
* {@link #remove remove()} |
* {@link #poll poll()} |
*
*
* | Examine |
* {@link #element element()} |
* {@link #peek peek()} |
*
*
*
* Queues typically, but do not necessarily, order elements in a
* FIFO (first-in-first-out) manner. Among the exceptions are
* priority queues, which order elements according to a supplied
* comparator, or the elements' natural ordering, 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 {@link #remove() } or
* {@link #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.
*
*
The {@link #offer offer} method inserts an element if possible,
* otherwise returning false. This differs from the {@link
* java.util.Collection#add Collection.add} method, which can fail to
* add an element only by throwing an unchecked exception. The
* offer method is designed for use when failure is a normal,
* rather than exceptional occurrence, for example, in fixed-capacity
* (or "bounded") queues.
*
*
The {@link #remove()} and {@link #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. The remove() and
* poll() methods differ only in their behavior when the
* queue is empty: the remove() method throws an exception,
* while the poll() method returns null.
*
*
The {@link #element()} and {@link #peek()} methods return, but do
* not remove, the head of the queue.
*
*
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 {@link java.util.concurrent.BlockingQueue} interface, which
* extends this interface.
*
*
Queue implementations generally do not allow insertion
* of null elements, although some implementations, such as
* {@link LinkedList}, do not prohibit insertion of null.
* 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.
*
*
Queue implementations generally do not define
* element-based versions of methods equals and
* hashCode but instead inherit the identity based versions
* from class Object, because element-based equality is not
* always well-defined for queues with the same elements but different
* ordering properties.
*
*
*
This interface is a member of the
*
* Java Collections Framework.
*
* @see java.util.Collection
* @see LinkedList
* @see PriorityQueue
* @see java.util.concurrent.LinkedBlockingQueue
* @see java.util.concurrent.BlockingQueue
* @see java.util.concurrent.ArrayBlockingQueue
* @see java.util.concurrent.LinkedBlockingQueue
* @see java.util.concurrent.PriorityBlockingQueue
* @since 1.5
* @author Doug Lea
* @param the type of elements held in this collection
*/
public interface Queue extends Collection {
/**
* 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.
*
* @param e the element to add
* @return 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 true if the element was added to this queue, else
* 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 null if this queue is empty.
*
* @return the head of this queue, or 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 null if this queue is empty.
*
* @return the head of this queue, or null if this queue is empty
*/
E peek();
}