20 |
|
* which would be removed by a call to {@link #remove() } or {@link #poll()}. |
21 |
|
* Every <tt>Queue</tt> implementation must specify its ordering guarantees. |
22 |
|
* |
23 |
< |
* <p>The {@link #offer offer(E)} method adds an element if possible, otherwise |
24 |
< |
* returning <tt>false</tt>. This differs from the {@link java.util.Collection#add Collection.add(E)} |
23 |
> |
* <p>The {@link #offer offer} method adds an element if possible, otherwise |
24 |
> |
* returning <tt>false</tt>. This differs from the |
25 |
> |
* {@link java.util.Collection#add Collection.add} |
26 |
|
* method, which throws an unchecked exception upon |
27 |
|
* failure. It is designed for use in collections in which failure to |
28 |
|
* add is a normal, rather than exceptional occurrence, for example, |
54 |
|
* used as a special return value by the <tt>poll</tt> method to |
55 |
|
* indicate that the queue contains no elements. |
56 |
|
* |
57 |
+ |
* <p><tt>Queue</tt> implementations generally do not define |
58 |
+ |
* element-based versions of methods <tt>equals</tt> and |
59 |
+ |
* <tt>hashCode</tt> but instead inherit the identity based versions |
60 |
+ |
* from class <tt>Object</tt>, because element-based equality is not |
61 |
+ |
* always well-defined for queues with the same elements but different |
62 |
+ |
* ordering properties. |
63 |
+ |
* |
64 |
+ |
* |
65 |
|
* <p>This interface is a member of the |
66 |
|
* <a href="{@docRoot}/../guide/collections/index.html"> |
67 |
|
* Java Collections Framework</a>. |
69 |
|
* @see java.util.Collection |
70 |
|
* @see LinkedList |
71 |
|
* @see PriorityQueue |
72 |
< |
* @see java.util.concurrent.LinkedQueue |
72 |
> |
* @see java.util.concurrent.LinkedBlockingQueue |
73 |
|
* @see java.util.concurrent.BlockingQueue |
74 |
|
* @see java.util.concurrent.ArrayBlockingQueue |
75 |
|
* @see java.util.concurrent.LinkedBlockingQueue |
80 |
|
public interface Queue<E> extends Collection<E> { |
81 |
|
|
82 |
|
/** |
83 |
< |
* Add the specified element to this queue, if possible. |
83 |
> |
* Adds the specified element to this queue, if possible. |
84 |
|
* |
85 |
|
* @param o the element to add. |
86 |
|
* @return <tt>true</tt> if it was possible to add the element to |
89 |
|
boolean offer(E o); |
90 |
|
|
91 |
|
/** |
92 |
< |
* Retrieve and remove the head of this queue, if it is available. |
92 |
> |
* Retrieves and removes the head of this queue, if it is available. |
93 |
|
* |
94 |
|
* @return the head of this queue, or <tt>null</tt> if this |
95 |
|
* queue is empty. |
97 |
|
E poll(); |
98 |
|
|
99 |
|
/** |
100 |
< |
* Retrieve and remove the head of this queue. |
100 |
> |
* Retrieves and removes the head of this queue. |
101 |
|
* This method differs |
102 |
|
* from the <tt>poll</tt> method in that it throws an exception if this |
103 |
|
* queue is empty. |
108 |
|
E remove(); |
109 |
|
|
110 |
|
/** |
111 |
< |
* Retrieve, but do not remove, the head of this queue. |
111 |
> |
* Retrieves, but does not remove, the head of this queue. |
112 |
|
* This method differs from the <tt>poll</tt> |
113 |
|
* method only in that this method does not remove the head element from |
114 |
|
* this queue. |
118 |
|
E peek(); |
119 |
|
|
120 |
|
/** |
121 |
< |
* Retrieve, but do not remove, the head of this queue. This method |
121 |
> |
* Retrieves, but does not remove, the head of this queue. This method |
122 |
|
* differs from the <tt>peek</tt> method only in that it throws an |
123 |
|
* exception if this queue is empty. |
124 |
|
* |