--- jsr166/src/main/java/util/Queue.java 2003/06/24 14:34:30 1.7 +++ jsr166/src/main/java/util/Queue.java 2016/07/08 20:02:54 1.44 @@ -1,118 +1,180 @@ /* * Written by Doug Lea with assistance from members of JCP JSR-166 - * Expert Group and released to the public domain. Use, modify, and - * redistribute this code in any way without acknowledgement. + * Expert Group and released to the public domain, as explained at + * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util; /** - * A Collection designed for holding elements prior to processing. - * Besides basic {@link Collection} operations, queues provide - * additional insertion, extraction, and inspection operations. -0 * + * 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 {@code null} or {@code false}, depending on the + * operation). The latter form of the insert operation is designed + * specifically for use with capacity-restricted {@code Queue} + * implementations; in most implementations, insert operations cannot + * fail. + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + * + *
Summary of Queue methods
Throws exceptionReturns special value
Insert{@link Queue#add add(e)}{@link Queue#offer offer(e)}
Remove{@link Queue#remove remove()}{@link Queue#poll poll()}
Examine{@link Queue#element element()}{@link Queue#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. Every Queue - * implementation must specify its ordering guarantees. + * 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 {@code Queue} implementation + * must specify its ordering properties. + * + *

The {@link #offer offer} method inserts an element if possible, + * otherwise returning {@code 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 + * {@code 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 #offer(E)} method adds an element if possible, otherwise - * returning false. This differs from the {@link - * Collections#add(Object)} method, which throws an unchecked exception upon - * failure. It is designed for use in collections in which failure to - * add 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 an element in accord with the implementation's ordering - * policy. Exactly which element is removed from the queue is a + * 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. Possible orderings include (but - * are not limited to) first-in-first-out (FIFO), last-in-first-out - * (LIFO), element priority, and arbitrary. 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 delete the element that would be obtained by a call to - * the remove and poll methods respectively. + * implementation to implementation. The {@code remove()} and + * {@code poll()} methods differ only in their behavior when the + * queue is empty: the {@code remove()} method throws an exception, + * while the {@code poll()} method returns {@code 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 + *

The {@code 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 + *

{@code Queue} implementations generally do not allow insertion + * of {@code null} elements, although some implementations, such as + * {@link LinkedList}, do not prohibit insertion of {@code null}. + * Even in the implementations that permit it, {@code null} should + * not be inserted into a {@code Queue}, as {@code null} is also + * used as a special return value by the {@code poll} method to * indicate that the queue contains no elements. * + *

{@code Queue} implementations generally do not define + * element-based versions of methods {@code equals} and + * {@code hashCode} but instead inherit the identity based versions + * from class {@code 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 Collection - * @see LinkedList - * @see PriorityQueue - * @see java.util.concurrent.LinkedQueue - * @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 queue */ public interface Queue extends Collection { /** - * Add the specified element to this queue, if possible. + * 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. * - * @param element the element to add. - * @return true if it was possible to add the element to the queue. + * @param e the element to add + * @return {@code 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 offer(E element); + boolean add(E e); /** - * Remove and return an element from the queue if one is available. + * 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. * - * @return an element previously on the queue, or null if the - * queue is empty. + * @param e the element to add + * @return {@code true} if the element was added to this queue, else + * {@code 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 */ - E poll(); + boolean offer(E e); /** - * Remove and return an element from the queue. This method differs - * from the poll method in that it throws an exception if the - * queue is empty. + * 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 an element previously on the queue. - * @throws NoSuchElementException if the queue is empty. + * @return the head of this queue + * @throws NoSuchElementException if this queue is empty */ - E remove() throws NoSuchElementException; + E remove(); /** - * Return, but do not remove, an element from the queue, or null - * if the queue is empty. This method returns the same object reference - * that would be returned by by the poll method. The two methods - * differ in that this method does not remove the element from the queue. + * Retrieves and removes the head of this queue, + * or returns {@code null} if this queue is empty. * - * @return an element on the queue, or null if the queue is empty. + * @return the head of this queue, or {@code null} if this queue is empty */ - E peek(); + 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(); /** - * Return, but do not remove, an element from the queue. This method - * differs from the peek method in that it throws an exception if - * the queue is empty. + * Retrieves, but does not remove, the head of this queue, + * or returns {@code null} if this queue is empty. * - * @return an element on the queue. - * @throws NoSuchElementException if the queue is empty. + * @return the head of this queue, or {@code null} if this queue is empty */ - E element() throws NoSuchElementException; + E peek(); }