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36 package java.util.concurrent;
39 * A {@link BlockingQueue} in which producers may wait for consumers
40 * to receive elements. A {@code TransferQueue} may be useful for
41 * example in message passing applications in which producers
42 * sometimes (using method {@link #transfer}) await receipt of
43 * elements by consumers invoking {@code take} or {@code poll}, while
44 * at other times enqueue elements (via method {@code put}) without
45 * waiting for receipt.
46 * {@linkplain #tryTransfer(Object) Non-blocking} and
47 * {@linkplain #tryTransfer(Object,long,TimeUnit) time-out} versions of
48 * {@code tryTransfer} are also available.
49 * A {@code TransferQueue} may also be queried, via {@link
50 * #hasWaitingConsumer}, whether there are any threads waiting for
51 * items, which is a converse analogy to a {@code peek} operation.
53 * <p>Like other blocking queues, a {@code TransferQueue} may be
54 * capacity bounded. If so, an attempted transfer operation may
55 * initially block waiting for available space, and/or subsequently
56 * block waiting for reception by a consumer. Note that in a queue
57 * with zero capacity, such as {@link SynchronousQueue}, {@code put}
58 * and {@code transfer} are effectively synonymous.
60 * <p>This interface is a member of the
61 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
62 * Java Collections Framework</a>.
66 * @param <E> the type of elements held in this collection
68 public interface TransferQueue<E> extends BlockingQueue<E> {
70 * Transfers the element to a waiting consumer immediately, if possible.
72 * <p>More precisely, transfers the specified element immediately
73 * if there exists a consumer already waiting to receive it (in
74 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
75 * otherwise returning {@code false} without enqueuing the element.
77 * @param e the element to transfer
78 * @return {@code true} if the element was transferred, else
80 * @throws ClassCastException if the class of the specified element
81 * prevents it from being added to this queue
82 * @throws NullPointerException if the specified element is null
83 * @throws IllegalArgumentException if some property of the specified
84 * element prevents it from being added to this queue
86 boolean tryTransfer(E e);
89 * Transfers the element to a consumer, waiting if necessary to do so.
91 * <p>More precisely, transfers the specified element immediately
92 * if there exists a consumer already waiting to receive it (in
93 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
94 * else waits until the element is received by a consumer.
96 * @param e the element to transfer
97 * @throws InterruptedException if interrupted while waiting,
98 * in which case the element is not left enqueued
99 * @throws ClassCastException if the class of the specified element
100 * prevents it from being added to this queue
101 * @throws NullPointerException if the specified element is null
102 * @throws IllegalArgumentException if some property of the specified
103 * element prevents it from being added to this queue
105 void transfer(E e) throws InterruptedException;
108 * Transfers the element to a consumer if it is possible to do so
109 * before the timeout elapses.
111 * <p>More precisely, transfers the specified element immediately
112 * if there exists a consumer already waiting to receive it (in
113 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
114 * else waits until the element is received by a consumer,
115 * returning {@code false} if the specified wait time elapses
116 * before the element can be transferred.
118 * @param e the element to transfer
119 * @param timeout how long to wait before giving up, in units of
121 * @param unit a {@code TimeUnit} determining how to interpret the
122 * {@code timeout} parameter
123 * @return {@code true} if successful, or {@code false} if
124 * the specified waiting time elapses before completion,
125 * in which case the element is not left enqueued
126 * @throws InterruptedException if interrupted while waiting,
127 * in which case the element is not left enqueued
128 * @throws ClassCastException if the class of the specified element
129 * prevents it from being added to this queue
130 * @throws NullPointerException if the specified element is null
131 * @throws IllegalArgumentException if some property of the specified
132 * element prevents it from being added to this queue
134 boolean tryTransfer(E e, long timeout, TimeUnit unit)
135 throws InterruptedException;
138 * Returns {@code true} if there is at least one consumer waiting
139 * to receive an element via {@link #take} or
140 * timed {@link #poll(long,TimeUnit) poll}.
141 * The return value represents a momentary state of affairs.
143 * @return {@code true} if there is at least one waiting consumer
145 boolean hasWaitingConsumer();
148 * Returns an estimate of the number of consumers waiting to
149 * receive elements via {@link #take} or timed
150 * {@link #poll(long,TimeUnit) poll}. The return value is an
151 * approximation of a momentary state of affairs, that may be
152 * inaccurate if consumers have completed or given up waiting.
153 * The value may be useful for monitoring and heuristics, but
154 * not for synchronization control. Implementations of this
155 * method are likely to be noticeably slower than those for
156 * {@link #hasWaitingConsumer}.
158 * @return the number of consumers waiting to receive elements
160 int getWaitingConsumerCount();