16 |
|
* priority queues, which order elements according to a supplied |
17 |
|
* comparator, or the elements' natural ordering, and LIFO queues (or |
18 |
|
* stacks) which order the elements LIFO (last-in-first-out). |
19 |
< |
* Whatever the ordering used, the <em>head</em> of the queue is that element |
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)} |
25 |
< |
* method, which throws an unchecked exception upon |
26 |
< |
* failure. It is designed for use in collections in which failure to |
27 |
< |
* add is a normal, rather than exceptional occurrence, for example, |
28 |
< |
* in fixed-capacity (or "bounded") queues. |
19 |
> |
* Whatever the ordering used, the <em>head</em> of the queue is that |
20 |
> |
* element which would be removed by a call to {@link #remove() } or |
21 |
> |
* {@link #poll()}. In a FIFO queue, all new elements are inserted at |
22 |
> |
* the <em> tail</em> of the queue. Other kinds of queues may use |
23 |
> |
* different placement rules. Every <tt>Queue</tt> implementation |
24 |
> |
* must specify its ordering properties. |
25 |
> |
* |
26 |
> |
* <p>The {@link #offer offer} method inserts an element if possible, |
27 |
> |
* otherwise returning <tt>false</tt>. This differs from the {@link |
28 |
> |
* java.util.Collection#add Collection.add} method, which can fail to |
29 |
> |
* add an element only by throwing an unchecked exception. The |
30 |
> |
* <tt>offer</tt> method is designed for use when failure is a normal, |
31 |
> |
* rather than exceptional occurrence, for example, in fixed-capacity |
32 |
> |
* (or "bounded") queues. |
33 |
|
* |
34 |
|
* <p>The {@link #remove()} and {@link #poll()} methods remove and |
35 |
|
* return the head of the queue. |
57 |
|
* used as a special return value by the <tt>poll</tt> method to |
58 |
|
* indicate that the queue contains no elements. |
59 |
|
* |
60 |
+ |
* <p><tt>Queue</tt> implementations generally do not define |
61 |
+ |
* element-based versions of methods <tt>equals</tt> and |
62 |
+ |
* <tt>hashCode</tt> but instead inherit the identity based versions |
63 |
+ |
* from class <tt>Object</tt>, because element-based equality is not |
64 |
+ |
* always well-defined for queues with the same elements but different |
65 |
+ |
* ordering properties. |
66 |
+ |
* |
67 |
+ |
* |
68 |
|
* <p>This interface is a member of the |
69 |
|
* <a href="{@docRoot}/../guide/collections/index.html"> |
70 |
|
* Java Collections Framework</a>. |
72 |
|
* @see java.util.Collection |
73 |
|
* @see LinkedList |
74 |
|
* @see PriorityQueue |
75 |
< |
* @see java.util.concurrent.LinkedQueue |
75 |
> |
* @see java.util.concurrent.LinkedBlockingQueue |
76 |
|
* @see java.util.concurrent.BlockingQueue |
77 |
|
* @see java.util.concurrent.ArrayBlockingQueue |
78 |
|
* @see java.util.concurrent.LinkedBlockingQueue |
79 |
|
* @see java.util.concurrent.PriorityBlockingQueue |
80 |
|
* @since 1.5 |
81 |
|
* @author Doug Lea |
82 |
+ |
* @param <E> the type of elements held in this collection |
83 |
|
*/ |
84 |
|
public interface Queue<E> extends Collection<E> { |
85 |
|
|
86 |
|
/** |
87 |
< |
* Add the specified element to this queue, if possible. |
87 |
> |
* Inserts the specified element into this queue, if possible. When |
88 |
> |
* using queues that may impose insertion restrictions (for |
89 |
> |
* example capacity bounds), method <tt>offer</tt> is generally |
90 |
> |
* preferable to method {@link Collection#add}, which can fail to |
91 |
> |
* insert an element only by throwing an exception. |
92 |
|
* |
93 |
< |
* @param o the element to add. |
93 |
> |
* @param o the element to insert. |
94 |
|
* @return <tt>true</tt> if it was possible to add the element to |
95 |
|
* this queue, else <tt>false</tt> |
96 |
|
*/ |
97 |
|
boolean offer(E o); |
98 |
|
|
99 |
|
/** |
100 |
< |
* Retrieve and remove the head of this queue, if it is available. |
100 |
> |
* Retrieves and removes the head of this queue, or <tt>null</tt> |
101 |
> |
* if this queue is empty. |
102 |
|
* |
103 |
|
* @return the head of this queue, or <tt>null</tt> if this |
104 |
|
* queue is empty. |
106 |
|
E poll(); |
107 |
|
|
108 |
|
/** |
109 |
< |
* Retrieve and remove the head of this queue. |
110 |
< |
* This method differs |
111 |
< |
* from the <tt>poll</tt> method in that it throws an exception if this |
94 |
< |
* queue is empty. |
109 |
> |
* Retrieves and removes the head of this queue. This method |
110 |
> |
* differs from the <tt>poll</tt> method in that it throws an |
111 |
> |
* exception if this queue is empty. |
112 |
|
* |
113 |
|
* @return the head of this queue. |
114 |
|
* @throws NoSuchElementException if this queue is empty. |
116 |
|
E remove(); |
117 |
|
|
118 |
|
/** |
119 |
< |
* Retrieve, but do not remove, the head of this queue. |
120 |
< |
* This method differs from the <tt>poll</tt> |
104 |
< |
* method only in that this method does not remove the head element from |
105 |
< |
* this queue. |
119 |
> |
* Retrieves, but does not remove, the head of this queue, |
120 |
> |
* returning <tt>null</tt> if this queue is empty. |
121 |
|
* |
122 |
< |
* @return the head of this queue, or <tt>null</tt> if this queue is empty. |
122 |
> |
* @return the head of this queue, or <tt>null</tt> if this queue |
123 |
> |
* is empty. |
124 |
|
*/ |
125 |
|
E peek(); |
126 |
|
|
127 |
|
/** |
128 |
< |
* Retrieve, but do not remove, the head of this queue. This method |
128 |
> |
* Retrieves, but does not remove, the head of this queue. This method |
129 |
|
* differs from the <tt>peek</tt> method only in that it throws an |
130 |
|
* exception if this queue is empty. |
131 |
|
* |
134 |
|
*/ |
135 |
|
E element(); |
136 |
|
} |
121 |
– |
|
122 |
– |
|
123 |
– |
|
124 |
– |
|
125 |
– |
|
126 |
– |
|
127 |
– |
|
128 |
– |
|
129 |
– |
|
130 |
– |
|