12 |
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* 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 |
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* fail. |
20 |
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* |
21 |
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* <p> |
22 |
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* <table BORDER CELLPADDING=3 CELLSPACING=1> |
23 |
+ |
* <caption>Summary of Queue methods</caption> |
24 |
|
* <tr> |
25 |
|
* <td></td> |
26 |
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* <td ALIGN=CENTER><em>Throws exception</em></td> |
28 |
|
* </tr> |
29 |
|
* <tr> |
30 |
|
* <td><b>Insert</b></td> |
31 |
< |
* <td>{@link #add add(e)}</td> |
32 |
< |
* <td>{@link #offer offer(e)}</td> |
31 |
> |
* <td>{@link Queue#add add(e)}</td> |
32 |
> |
* <td>{@link Queue#offer offer(e)}</td> |
33 |
|
* </tr> |
34 |
|
* <tr> |
35 |
|
* <td><b>Remove</b></td> |
36 |
< |
* <td>{@link #remove remove()}</td> |
37 |
< |
* <td>{@link #poll poll()}</td> |
36 |
> |
* <td>{@link Queue#remove remove()}</td> |
37 |
> |
* <td>{@link Queue#poll poll()}</td> |
38 |
|
* </tr> |
39 |
|
* <tr> |
40 |
|
* <td><b>Examine</b></td> |
41 |
< |
* <td>{@link #element element()}</td> |
42 |
< |
* <td>{@link #peek peek()}</td> |
41 |
> |
* <td>{@link Queue#element element()}</td> |
42 |
> |
* <td>{@link Queue#peek peek()}</td> |
43 |
|
* </tr> |
44 |
|
* </table> |
45 |
|
* |
52 |
|
* element which would be removed by a call to {@link #remove() } or |
53 |
|
* {@link #poll()}. In a FIFO queue, all new elements are inserted at |
54 |
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* the <em>tail</em> of the queue. Other kinds of queues may use |
55 |
< |
* different placement rules. Every <tt>Queue</tt> implementation |
55 |
> |
* different placement rules. Every {@code Queue} implementation |
56 |
|
* must specify its ordering properties. |
57 |
|
* |
58 |
|
* <p>The {@link #offer offer} method inserts an element if possible, |
59 |
< |
* otherwise returning <tt>false</tt>. This differs from the {@link |
59 |
> |
* otherwise returning {@code false}. This differs from the {@link |
60 |
|
* java.util.Collection#add Collection.add} method, which can fail to |
61 |
|
* add an element only by throwing an unchecked exception. The |
62 |
< |
* <tt>offer</tt> method is designed for use when failure is a normal, |
62 |
> |
* {@code offer} method is designed for use when failure is a normal, |
63 |
|
* rather than exceptional occurrence, for example, in fixed-capacity |
64 |
|
* (or "bounded") queues. |
65 |
|
* |
67 |
|
* return the head of the queue. |
68 |
|
* Exactly which element is removed from the queue is a |
69 |
|
* function of the queue's ordering policy, which differs from |
70 |
< |
* implementation to implementation. The <tt>remove()</tt> and |
71 |
< |
* <tt>poll()</tt> methods differ only in their behavior when the |
72 |
< |
* queue is empty: the <tt>remove()</tt> method throws an exception, |
73 |
< |
* while the <tt>poll()</tt> method returns <tt>null</tt>. |
70 |
> |
* implementation to implementation. The {@code remove()} and |
71 |
> |
* {@code poll()} methods differ only in their behavior when the |
72 |
> |
* queue is empty: the {@code remove()} method throws an exception, |
73 |
> |
* while the {@code poll()} method returns {@code null}. |
74 |
|
* |
75 |
|
* <p>The {@link #element()} and {@link #peek()} methods return, but do |
76 |
|
* not remove, the head of the queue. |
77 |
|
* |
78 |
< |
* <p>The <tt>Queue</tt> interface does not define the <i>blocking queue |
78 |
> |
* <p>The {@code Queue} interface does not define the <i>blocking queue |
79 |
|
* methods</i>, which are common in concurrent programming. These methods, |
80 |
|
* which wait for elements to appear or for space to become available, are |
81 |
|
* defined in the {@link java.util.concurrent.BlockingQueue} interface, which |
82 |
|
* extends this interface. |
83 |
|
* |
84 |
< |
* <p><tt>Queue</tt> implementations generally do not allow insertion |
85 |
< |
* of <tt>null</tt> elements, although some implementations, such as |
86 |
< |
* {@link LinkedList}, do not prohibit insertion of <tt>null</tt>. |
87 |
< |
* Even in the implementations that permit it, <tt>null</tt> should |
88 |
< |
* not be inserted into a <tt>Queue</tt>, as <tt>null</tt> is also |
89 |
< |
* used as a special return value by the <tt>poll</tt> method to |
84 |
> |
* <p>{@code Queue} implementations generally do not allow insertion |
85 |
> |
* of {@code null} elements, although some implementations, such as |
86 |
> |
* {@link LinkedList}, do not prohibit insertion of {@code null}. |
87 |
> |
* Even in the implementations that permit it, {@code null} should |
88 |
> |
* not be inserted into a {@code Queue}, as {@code null} is also |
89 |
> |
* used as a special return value by the {@code poll} method to |
90 |
|
* indicate that the queue contains no elements. |
91 |
|
* |
92 |
< |
* <p><tt>Queue</tt> implementations generally do not define |
93 |
< |
* element-based versions of methods <tt>equals</tt> and |
94 |
< |
* <tt>hashCode</tt> but instead inherit the identity based versions |
95 |
< |
* from class <tt>Object</tt>, because element-based equality is not |
92 |
> |
* <p>{@code Queue} implementations generally do not define |
93 |
> |
* element-based versions of methods {@code equals} and |
94 |
> |
* {@code hashCode} but instead inherit the identity based versions |
95 |
> |
* from class {@code Object}, because element-based equality is not |
96 |
|
* always well-defined for queues with the same elements but different |
97 |
|
* ordering properties. |
98 |
|
* |
117 |
|
/** |
118 |
|
* Inserts the specified element into this queue if it is possible to do so |
119 |
|
* immediately without violating capacity restrictions, returning |
120 |
< |
* <tt>true</tt> upon success and throwing an <tt>IllegalStateException</tt> |
120 |
> |
* {@code true} upon success and throwing an {@code IllegalStateException} |
121 |
|
* if no space is currently available. |
122 |
|
* |
123 |
|
* @param e the element to add |
124 |
< |
* @return <tt>true</tt> (as specified by {@link Collection#add}) |
124 |
> |
* @return {@code true} (as specified by {@link Collection#add}) |
125 |
|
* @throws IllegalStateException if the element cannot be added at this |
126 |
|
* time due to capacity restrictions |
127 |
|
* @throws ClassCastException if the class of the specified element |
141 |
|
* by throwing an exception. |
142 |
|
* |
143 |
|
* @param e the element to add |
144 |
< |
* @return <tt>true</tt> if the element was added to this queue, else |
145 |
< |
* <tt>false</tt> |
144 |
> |
* @return {@code true} if the element was added to this queue, else |
145 |
> |
* {@code false} |
146 |
|
* @throws ClassCastException if the class of the specified element |
147 |
|
* prevents it from being added to this queue |
148 |
|
* @throws NullPointerException if the specified element is null and |
164 |
|
|
165 |
|
/** |
166 |
|
* Retrieves and removes the head of this queue, |
167 |
< |
* or returns <tt>null</tt> if this queue is empty. |
167 |
> |
* or returns {@code null} if this queue is empty. |
168 |
|
* |
169 |
< |
* @return the head of this queue, or <tt>null</tt> if this queue is empty |
169 |
> |
* @return the head of this queue, or {@code null} if this queue is empty |
170 |
|
*/ |
171 |
|
E poll(); |
172 |
|
|
182 |
|
|
183 |
|
/** |
184 |
|
* Retrieves, but does not remove, the head of this queue, |
185 |
< |
* or returns <tt>null</tt> if this queue is empty. |
185 |
> |
* or returns {@code null} if this queue is empty. |
186 |
|
* |
187 |
< |
* @return the head of this queue, or <tt>null</tt> if this queue is empty |
187 |
> |
* @return the head of this queue, or {@code null} if this queue is empty |
188 |
|
*/ |
189 |
|
E peek(); |
190 |
|
} |