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; |
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 <tt>null</tt> or <tt>false</tt>, depending on the |
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 <tt>Queue</tt> |
17 |
> |
* specifically for use with capacity-restricted {@code Queue} |
18 |
|
* implementations; in most implementations, insert operations cannot |
19 |
|
* fail. |
20 |
|
* |
21 |
< |
* <p><table BORDER CELLPADDING=3 CELLSPACING=1> |
21 |
> |
* <p> |
22 |
> |
* <table BORDER CELLPADDING=3 CELLSPACING=1> |
23 |
|
* <tr> |
24 |
|
* <td></td> |
25 |
|
* <td ALIGN=CENTER><em>Throws exception</em></td> |
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> { |
115 |
– |
|
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 |
121 |
< |
* insert an element only by throwing an exception. |
122 |
< |
* |
123 |
< |
* @param e the element to insert. |
124 |
< |
* @return <tt>true</tt> if it was possible to add the element to |
125 |
< |
* this queue, else <tt>false</tt> |
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 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 e); |
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 {@link #poll} method only 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 |
153 |
< |
* 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 {@link #peek} 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 |
|
} |