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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/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 <tt>null</tt> or <tt>false</tt>, depending on the
* operation). The latter form of the insert operation is designed
* specifically for use with capacity-restricted <tt>Queue</tt>
* implementations; in most implementations, insert operations cannot
* fail.
*
* <p>
* <table BORDER CELLPADDING=3 CELLSPACING=1>
* <tr>
* <td></td>
* <td ALIGN=CENTER><em>Throws exception</em></td>
* <td ALIGN=CENTER><em>Returns special value</em></td>
* </tr>
* <tr>
* <td><b>Insert</b></td>
* <td>{@link #add add(e)}</td>
* <td>{@link #offer offer(e)}</td>
* </tr>
* <tr>
* <td><b>Remove</b></td>
* <td>{@link #remove remove()}</td>
* <td>{@link #poll poll()}</td>
* </tr>
* <tr>
* <td><b>Examine</b></td>
* <td>{@link #element element()}</td>
* <td>{@link #peek peek()}</td>
* </tr>
* </table>
*
* <p>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 <em>head</em> 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 <em> tail</em> of the queue. Other kinds of queues may use
* different placement rules. Every <tt>Queue</tt> implementation
* must specify its ordering properties.
*
* <p>The {@link #offer offer} method inserts an element if possible,
* otherwise returning <tt>false</tt>. 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
* <tt>offer</tt> method is designed for use when failure is a normal,
* rather than exceptional occurrence, for example, in fixed-capacity
* (or "bounded") queues.
*
* <p>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 <tt>remove()</tt> and
* <tt>poll()</tt> methods differ only in their behavior when the
* queue is empty: the <tt>remove()</tt> method throws an exception,
* while the <tt>poll()</tt> method returns <tt>null</tt>.
*
* <p>The {@link #element()} and {@link #peek()} methods return, but do
* not remove, the head of the queue.
*
* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue
* methods</i>, 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.
*
* <p><tt>Queue</tt> implementations generally do not allow insertion
* of <tt>null</tt> elements, although some implementations, such as
* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>.
* Even in the implementations that permit it, <tt>null</tt> should
* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also
* used as a special return value by the <tt>poll</tt> method to
* indicate that the queue contains no elements.
*
* <p><tt>Queue</tt> implementations generally do not define
* element-based versions of methods <tt>equals</tt> and
* <tt>hashCode</tt> but instead inherit the identity based versions
* from class <tt>Object</tt>, because element-based equality is not
* always well-defined for queues with the same elements but different
* ordering properties.
*
*
* <p>This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @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 <E> the type of elements held in this collection
*/
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
* <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt>
* if no space is currently available.
*
* @param e the element to add
* @return <tt>true</tt> (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 <tt>true</tt> if the element was added to this queue, else
* <tt>false</tt>
* @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 <tt>null</tt> if this queue is empty.
*
* @return the head of this queue, or <tt>null</tt> 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 <tt>null</tt> if this queue is empty.
*
* @return the head of this queue, or <tt>null</tt> if this queue is empty
*/
E peek();
}
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