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/publicdomain/zero/1.0/ |
5 |
+ |
*/ |
6 |
+ |
|
7 |
|
package java.util; |
8 |
|
|
9 |
|
/** |
10 |
< |
* Queues are Collections supporting additional basic insertion, |
11 |
< |
* extraction, and inspection operations. |
12 |
< |
* |
13 |
< |
* <p> Queues typically, but do not necessarily order elements in a |
14 |
< |
* FIFO (first-in-first-out) manner. Among the exceptions are priority |
15 |
< |
* queues, that order elements in accord with supplied |
16 |
< |
* Comparators. Every Queue implementation must specify its ordering |
17 |
< |
* guarantees, |
18 |
< |
* |
19 |
< |
* <p> The <tt>offer</tt> method adds an element if possible, |
20 |
< |
* otherwise returning <tt>false</tt>. This differs from the |
21 |
< |
* Collections.add method, that throws an unchecked exception upon |
22 |
< |
* failure. It is designed for use in collections in which failure to |
23 |
< |
* add is a normal, rather than exceptional occurrence, for example, |
24 |
< |
* in fixed-capacity queues. |
25 |
< |
* |
26 |
< |
* <p> The <tt>remove</tt> and <tt>poll</tt> methods delete and return |
27 |
< |
* an element in accord with the implementation's ordering policies -- |
28 |
< |
* for example, in FIFO queues, it will return the oldest element. |
29 |
< |
* The <tt>remove</tt> and <tt>poll</tt> differ only in their behavior |
30 |
< |
* when the queue is empty: <tt>poll</tt> returns <tt>null</tt> while |
31 |
< |
* <tt>remove</tt> throws an exception. These are designed for usage |
32 |
< |
* contexts in which emptiness is considered to be normal versus |
33 |
< |
* exceptional. |
34 |
< |
* |
35 |
< |
* <p> The <tt>element</tt> and <tt>peek</tt> methods return but do |
36 |
< |
* not delete the element that would be obtained by a call to |
37 |
< |
* <tt>remove</tt> and <tt>poll</tt> respectively. |
38 |
< |
* |
39 |
< |
* <p> The Queue interface does not define blocking queue methods |
40 |
< |
* (i.e., those that wait for elements to appear and/or for space to |
41 |
< |
* be available) that are common in concurrent programming. These are |
42 |
< |
* defined in the extended java.util.concurrent.BlockingQueue |
43 |
< |
* interface. |
44 |
< |
* |
45 |
< |
* <p> Queue implementations generally do not allow insertion of |
46 |
< |
* <tt>null</tt>. Even in those that allow it, it is a very bad idea |
47 |
< |
* to do so, since <tt>null</tt> is also used as a sentinel by |
48 |
< |
* <tt>poll</tt> to indicate that no elements exist. |
49 |
< |
**/ |
10 |
> |
* A collection designed for holding elements prior to processing. |
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 |
47 |
> |
* priority queues, which order elements according to a supplied |
48 |
> |
* comparator, or the elements' natural ordering, and LIFO queues (or |
49 |
> |
* stacks) which order the elements LIFO (last-in-first-out). |
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 {@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 {@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 |
> |
* {@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 |
> |
* |
65 |
> |
* <p>The {@link #remove()} and {@link #poll()} methods remove and |
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 {@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 {@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>{@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>{@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}/../technotes/guides/collections/index.html"> |
101 |
> |
* Java Collections Framework</a>. |
102 |
> |
* |
103 |
> |
* @see java.util.Collection |
104 |
> |
* @see LinkedList |
105 |
> |
* @see PriorityQueue |
106 |
> |
* @see java.util.concurrent.LinkedBlockingQueue |
107 |
> |
* @see java.util.concurrent.BlockingQueue |
108 |
> |
* @see java.util.concurrent.ArrayBlockingQueue |
109 |
> |
* @see java.util.concurrent.LinkedBlockingQueue |
110 |
> |
* @see java.util.concurrent.PriorityBlockingQueue |
111 |
> |
* @since 1.5 |
112 |
> |
* @author Doug Lea |
113 |
> |
* @param <E> the type of elements held in this collection |
114 |
> |
*/ |
115 |
|
public interface Queue<E> extends Collection<E> { |
116 |
+ |
/** |
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 add(E e); |
134 |
+ |
|
135 |
+ |
/** |
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 |
+ |
* @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 |
+ |
boolean offer(E e); |
153 |
+ |
|
154 |
+ |
/** |
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 |
161 |
+ |
*/ |
162 |
+ |
E remove(); |
163 |
+ |
|
164 |
+ |
/** |
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 {@code null} if this queue is empty |
169 |
+ |
*/ |
170 |
+ |
E poll(); |
171 |
+ |
|
172 |
+ |
/** |
173 |
+ |
* Retrieves, but does not remove, the head of this queue. This method |
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 |
179 |
+ |
*/ |
180 |
+ |
E element(); |
181 |
|
|
182 |
|
/** |
183 |
< |
* Add the given object to this queue if possible. |
184 |
< |
* @param x the object to add |
185 |
< |
* @return true if successful |
186 |
< |
**/ |
187 |
< |
public boolean offer(E x); |
188 |
< |
|
53 |
< |
/** |
54 |
< |
* Delete and return an object from the queue if one is available. |
55 |
< |
* @return the object, or null if the queue is empty. |
56 |
< |
**/ |
57 |
< |
public E poll(); |
58 |
< |
|
59 |
< |
/** |
60 |
< |
* Delete and return the element produced by poll, if the queue is |
61 |
< |
* not empty. |
62 |
< |
* @return an element |
63 |
< |
* @throws NoSuchElementException if empty |
64 |
< |
**/ |
65 |
< |
public E remove() throws NoSuchElementException; |
66 |
< |
|
67 |
< |
/** |
68 |
< |
* Return but do not delete the element that will be returned by |
69 |
< |
* the next call to poll. |
70 |
< |
* @return an element, or null if empty |
71 |
< |
**/ |
72 |
< |
public E peek(); |
73 |
< |
|
74 |
< |
/** |
75 |
< |
* Return but do not delete the element that will be returned by |
76 |
< |
* the next call to poll, if the queue is not empty. |
77 |
< |
* @return an element |
78 |
< |
* @throws NoSuchElementException if empty |
79 |
< |
**/ |
80 |
< |
public E element() throws NoSuchElementException; |
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 |
|
} |