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/*
 * 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/publicdomain/zero/1.0/
 */

package java.util;

// BEGIN android-note
// removed link to collections framework docs
// END android-note

/**
 * A collection designed for holding elements prior to processing.
 * Besides basic {@link 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 exceptionReturns 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 * 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. * * @see 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(); }





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