| 1 |
|
/* |
| 2 |
|
* Written by Doug Lea with assistance from members of JCP JSR-166 |
| 3 |
|
* Expert Group and released to the public domain, as explained at |
| 4 |
< |
* http://creativecommons.org/licenses/publicdomain |
| 4 |
> |
* http://creativecommons.org/publicdomain/zero/1.0/ |
| 5 |
|
*/ |
| 6 |
|
|
| 7 |
|
package java.util; |
| 8 |
|
|
| 9 |
|
/** |
| 10 |
|
* A collection designed for holding elements prior to processing. |
| 11 |
< |
* Besides basic {@link java.util.Collection Collection} operations, queues provide |
| 12 |
< |
* additional insertion, extraction, and inspection operations. |
| 11 |
> |
* Besides basic {@link java.util.Collection Collection} operations, |
| 12 |
> |
* queues provide additional insertion, extraction, and inspection |
| 13 |
> |
* operations. Each of these methods exists in two forms: one throws |
| 14 |
> |
* an exception if the operation fails, the other returns a special |
| 15 |
> |
* value (either {@code null} or {@code false}, depending on the |
| 16 |
> |
* operation). The latter form of the insert operation is designed |
| 17 |
> |
* specifically for use with capacity-restricted {@code Queue} |
| 18 |
> |
* implementations; in most implementations, insert operations cannot |
| 19 |
> |
* fail. |
| 20 |
> |
* |
| 21 |
> |
* <p> |
| 22 |
> |
* <table BORDER CELLPADDING=3 CELLSPACING=1> |
| 23 |
> |
* <tr> |
| 24 |
> |
* <td></td> |
| 25 |
> |
* <td ALIGN=CENTER><em>Throws exception</em></td> |
| 26 |
> |
* <td ALIGN=CENTER><em>Returns special value</em></td> |
| 27 |
> |
* </tr> |
| 28 |
> |
* <tr> |
| 29 |
> |
* <td><b>Insert</b></td> |
| 30 |
> |
* <td>{@link #add add(e)}</td> |
| 31 |
> |
* <td>{@link #offer offer(e)}</td> |
| 32 |
> |
* </tr> |
| 33 |
> |
* <tr> |
| 34 |
> |
* <td><b>Remove</b></td> |
| 35 |
> |
* <td>{@link #remove remove()}</td> |
| 36 |
> |
* <td>{@link #poll poll()}</td> |
| 37 |
> |
* </tr> |
| 38 |
> |
* <tr> |
| 39 |
> |
* <td><b>Examine</b></td> |
| 40 |
> |
* <td>{@link #element element()}</td> |
| 41 |
> |
* <td>{@link #peek peek()}</td> |
| 42 |
> |
* </tr> |
| 43 |
> |
* </table> |
| 44 |
|
* |
| 45 |
|
* <p>Queues typically, but do not necessarily, order elements in a |
| 46 |
|
* FIFO (first-in-first-out) manner. Among the exceptions are |
| 50 |
|
* Whatever the ordering used, the <em>head</em> of the queue is that |
| 51 |
|
* element which would be removed by a call to {@link #remove() } or |
| 52 |
|
* {@link #poll()}. In a FIFO queue, all new elements are inserted at |
| 53 |
< |
* the <em> tail</em> of the queue. Other kinds of queues may use |
| 54 |
< |
* different placement rules. Every <tt>Queue</tt> implementation |
| 53 |
> |
* the <em>tail</em> of the queue. Other kinds of queues may use |
| 54 |
> |
* different placement rules. Every {@code Queue} implementation |
| 55 |
|
* must specify its ordering properties. |
| 56 |
|
* |
| 57 |
|
* <p>The {@link #offer offer} method inserts an element if possible, |
| 58 |
< |
* otherwise returning <tt>false</tt>. This differs from the {@link |
| 58 |
> |
* otherwise returning {@code false}. This differs from the {@link |
| 59 |
|
* java.util.Collection#add Collection.add} method, which can fail to |
| 60 |
|
* add an element only by throwing an unchecked exception. The |
| 61 |
< |
* <tt>offer</tt> method is designed for use when failure is a normal, |
| 61 |
> |
* {@code offer} method is designed for use when failure is a normal, |
| 62 |
|
* rather than exceptional occurrence, for example, in fixed-capacity |
| 63 |
|
* (or "bounded") queues. |
| 64 |
|
* |
| 66 |
|
* return the head of the queue. |
| 67 |
|
* Exactly which element is removed from the queue is a |
| 68 |
|
* function of the queue's ordering policy, which differs from |
| 69 |
< |
* implementation to implementation. The <tt>remove()</tt> and |
| 70 |
< |
* <tt>poll()</tt> methods differ only in their behavior when the |
| 71 |
< |
* queue is empty: the <tt>remove()</tt> method throws an exception, |
| 72 |
< |
* while the <tt>poll()</tt> method returns <tt>null</tt>. |
| 69 |
> |
* implementation to implementation. The {@code remove()} and |
| 70 |
> |
* {@code poll()} methods differ only in their behavior when the |
| 71 |
> |
* queue is empty: the {@code remove()} method throws an exception, |
| 72 |
> |
* while the {@code poll()} method returns {@code null}. |
| 73 |
|
* |
| 74 |
|
* <p>The {@link #element()} and {@link #peek()} methods return, but do |
| 75 |
|
* not remove, the head of the queue. |
| 76 |
|
* |
| 77 |
< |
* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue |
| 77 |
> |
* <p>The {@code Queue} interface does not define the <i>blocking queue |
| 78 |
|
* methods</i>, which are common in concurrent programming. These methods, |
| 79 |
|
* which wait for elements to appear or for space to become available, are |
| 80 |
|
* defined in the {@link java.util.concurrent.BlockingQueue} interface, which |
| 81 |
|
* extends this interface. |
| 82 |
|
* |
| 83 |
< |
* <p><tt>Queue</tt> implementations generally do not allow insertion |
| 84 |
< |
* of <tt>null</tt> elements, although some implementations, such as |
| 85 |
< |
* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>. |
| 86 |
< |
* Even in the implementations that permit it, <tt>null</tt> should |
| 87 |
< |
* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also |
| 88 |
< |
* used as a special return value by the <tt>poll</tt> method to |
| 83 |
> |
* <p>{@code Queue} implementations generally do not allow insertion |
| 84 |
> |
* of {@code null} elements, although some implementations, such as |
| 85 |
> |
* {@link LinkedList}, do not prohibit insertion of {@code null}. |
| 86 |
> |
* Even in the implementations that permit it, {@code null} should |
| 87 |
> |
* not be inserted into a {@code Queue}, as {@code null} is also |
| 88 |
> |
* used as a special return value by the {@code poll} method to |
| 89 |
|
* indicate that the queue contains no elements. |
| 90 |
|
* |
| 91 |
< |
* <p><tt>Queue</tt> implementations generally do not define |
| 92 |
< |
* element-based versions of methods <tt>equals</tt> and |
| 93 |
< |
* <tt>hashCode</tt> but instead inherit the identity based versions |
| 94 |
< |
* from class <tt>Object</tt>, because element-based equality is not |
| 91 |
> |
* <p>{@code Queue} implementations generally do not define |
| 92 |
> |
* element-based versions of methods {@code equals} and |
| 93 |
> |
* {@code hashCode} but instead inherit the identity based versions |
| 94 |
> |
* from class {@code Object}, because element-based equality is not |
| 95 |
|
* always well-defined for queues with the same elements but different |
| 96 |
|
* ordering properties. |
| 97 |
|
* |
| 98 |
|
* |
| 99 |
|
* <p>This interface is a member of the |
| 100 |
< |
* <a href="{@docRoot}/../guide/collections/index.html"> |
| 100 |
> |
* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
| 101 |
|
* Java Collections Framework</a>. |
| 102 |
|
* |
| 103 |
|
* @see java.util.Collection |
| 113 |
|
* @param <E> the type of elements held in this collection |
| 114 |
|
*/ |
| 115 |
|
public interface Queue<E> extends Collection<E> { |
| 85 |
– |
|
| 116 |
|
/** |
| 117 |
< |
* Inserts the specified element into this queue, if possible. When |
| 118 |
< |
* using queues that may impose insertion restrictions (for |
| 119 |
< |
* example capacity bounds), method <tt>offer</tt> is generally |
| 120 |
< |
* preferable to method {@link Collection#add}, which can fail to |
| 91 |
< |
* insert an element only by throwing an exception. |
| 117 |
> |
* Inserts the specified element into this queue if it is possible to do so |
| 118 |
> |
* immediately without violating capacity restrictions, returning |
| 119 |
> |
* {@code true} upon success and throwing an {@code IllegalStateException} |
| 120 |
> |
* if no space is currently available. |
| 121 |
|
* |
| 122 |
< |
* @param o the element to insert. |
| 123 |
< |
* @return <tt>true</tt> if it was possible to add the element to |
| 124 |
< |
* this queue, else <tt>false</tt> |
| 122 |
> |
* @param e the element to add |
| 123 |
> |
* @return {@code true} (as specified by {@link Collection#add}) |
| 124 |
> |
* @throws IllegalStateException if the element cannot be added at this |
| 125 |
> |
* time due to capacity restrictions |
| 126 |
> |
* @throws ClassCastException if the class of the specified element |
| 127 |
> |
* prevents it from being added to this queue |
| 128 |
> |
* @throws NullPointerException if the specified element is null and |
| 129 |
> |
* this queue does not permit null elements |
| 130 |
> |
* @throws IllegalArgumentException if some property of this element |
| 131 |
> |
* prevents it from being added to this queue |
| 132 |
|
*/ |
| 133 |
< |
boolean offer(E o); |
| 133 |
> |
boolean add(E e); |
| 134 |
|
|
| 135 |
|
/** |
| 136 |
< |
* Retrieves and removes the head of this queue, or <tt>null</tt> |
| 137 |
< |
* if this queue is empty. |
| 136 |
> |
* Inserts the specified element into this queue if it is possible to do |
| 137 |
> |
* so immediately without violating capacity restrictions. |
| 138 |
> |
* When using a capacity-restricted queue, this method is generally |
| 139 |
> |
* preferable to {@link #add}, which can fail to insert an element only |
| 140 |
> |
* by throwing an exception. |
| 141 |
|
* |
| 142 |
< |
* @return the head of this queue, or <tt>null</tt> if this |
| 143 |
< |
* queue is empty. |
| 142 |
> |
* @param e the element to add |
| 143 |
> |
* @return {@code true} if the element was added to this queue, else |
| 144 |
> |
* {@code false} |
| 145 |
> |
* @throws ClassCastException if the class of the specified element |
| 146 |
> |
* prevents it from being added to this queue |
| 147 |
> |
* @throws NullPointerException if the specified element is null and |
| 148 |
> |
* this queue does not permit null elements |
| 149 |
> |
* @throws IllegalArgumentException if some property of this element |
| 150 |
> |
* prevents it from being added to this queue |
| 151 |
|
*/ |
| 152 |
< |
E poll(); |
| 152 |
> |
boolean offer(E e); |
| 153 |
|
|
| 154 |
|
/** |
| 155 |
< |
* Retrieves and removes the head of this queue. This method |
| 156 |
< |
* differs from the <tt>poll</tt> method in that it throws an |
| 157 |
< |
* exception if this queue is empty. |
| 155 |
> |
* Retrieves and removes the head of this queue. This method differs |
| 156 |
> |
* from {@link #poll poll} only in that it throws an exception if this |
| 157 |
> |
* queue is empty. |
| 158 |
|
* |
| 159 |
< |
* @return the head of this queue. |
| 160 |
< |
* @throws NoSuchElementException if this queue is empty. |
| 159 |
> |
* @return the head of this queue |
| 160 |
> |
* @throws NoSuchElementException if this queue is empty |
| 161 |
|
*/ |
| 162 |
|
E remove(); |
| 163 |
|
|
| 164 |
|
/** |
| 165 |
< |
* Retrieves, but does not remove, the head of this queue, |
| 166 |
< |
* returning <tt>null</tt> if this queue is empty. |
| 165 |
> |
* Retrieves and removes the head of this queue, |
| 166 |
> |
* or returns {@code null} if this queue is empty. |
| 167 |
|
* |
| 168 |
< |
* @return the head of this queue, or <tt>null</tt> if this queue |
| 123 |
< |
* is empty. |
| 168 |
> |
* @return the head of this queue, or {@code null} if this queue is empty |
| 169 |
|
*/ |
| 170 |
< |
E peek(); |
| 170 |
> |
E poll(); |
| 171 |
|
|
| 172 |
|
/** |
| 173 |
|
* Retrieves, but does not remove, the head of this queue. This method |
| 174 |
< |
* differs from the <tt>peek</tt> method only in that it throws an |
| 175 |
< |
* exception if this queue is empty. |
| 174 |
> |
* differs from {@link #peek peek} only in that it throws an exception |
| 175 |
> |
* if this queue is empty. |
| 176 |
|
* |
| 177 |
< |
* @return the head of this queue. |
| 178 |
< |
* @throws NoSuchElementException if this queue is empty. |
| 177 |
> |
* @return the head of this queue |
| 178 |
> |
* @throws NoSuchElementException if this queue is empty |
| 179 |
|
*/ |
| 180 |
|
E element(); |
| 181 |
+ |
|
| 182 |
+ |
/** |
| 183 |
+ |
* Retrieves, but does not remove, the head of this queue, |
| 184 |
+ |
* or returns {@code null} if this queue is empty. |
| 185 |
+ |
* |
| 186 |
+ |
* @return the head of this queue, or {@code null} if this queue is empty |
| 187 |
+ |
*/ |
| 188 |
+ |
E peek(); |
| 189 |
|
} |
