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779 lines
25 KiB
Java
779 lines
25 KiB
Java
/*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/licenses/publicdomain
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*/
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package java.util.concurrent;
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import java.util.concurrent.locks.*;
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import java.util.*;
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/**
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* A bounded {@linkplain BlockingQueue blocking queue} backed by an
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* array. This queue orders elements FIFO (first-in-first-out). The
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* <em>head</em> of the queue is that element that has been on the
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* queue the longest time. The <em>tail</em> of the queue is that
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* element that has been on the queue the shortest time. New elements
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* are inserted at the tail of the queue, and the queue retrieval
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* operations obtain elements at the head of the queue.
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*
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* <p>This is a classic "bounded buffer", in which a
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* fixed-sized array holds elements inserted by producers and
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* extracted by consumers. Once created, the capacity cannot be
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* increased. Attempts to <tt>put</tt> an element into a full queue
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* will result in the operation blocking; attempts to <tt>take</tt> an
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* element from an empty queue will similarly block.
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*
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* <p> This class supports an optional fairness policy for ordering
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* waiting producer and consumer threads. By default, this ordering
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* is not guaranteed. However, a queue constructed with fairness set
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* to <tt>true</tt> grants threads access in FIFO order. Fairness
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* generally decreases throughput but reduces variability and avoids
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* starvation.
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*
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* <p>This class and its iterator implement all of the
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* <em>optional</em> methods of the {@link Collection} and {@link
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* Iterator} interfaces.
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*
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* <p>This class is a member of the
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* <a href="{@docRoot}/../technotes/guides/collections/index.html">
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* Java Collections Framework</a>.
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*
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* @since 1.5
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* @author Doug Lea
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* @param <E> the type of elements held in this collection
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*/
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public class ArrayBlockingQueue<E> extends AbstractQueue<E>
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implements BlockingQueue<E>, java.io.Serializable {
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/**
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* Serialization ID. This class relies on default serialization
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* even for the items array, which is default-serialized, even if
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* it is empty. Otherwise it could not be declared final, which is
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* necessary here.
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*/
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private static final long serialVersionUID = -817911632652898426L;
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/** The queued items */
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private final E[] items;
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/** items index for next take, poll or remove */
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private int takeIndex;
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/** items index for next put, offer, or add. */
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private int putIndex;
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/** Number of items in the queue */
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private int count;
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/*
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* Concurrency control uses the classic two-condition algorithm
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* found in any textbook.
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*/
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/** Main lock guarding all access */
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private final ReentrantLock lock;
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/** Condition for waiting takes */
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private final Condition notEmpty;
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/** Condition for waiting puts */
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private final Condition notFull;
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// Internal helper methods
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/**
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* Circularly increment i.
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*/
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final int inc(int i) {
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return (++i == items.length)? 0 : i;
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}
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/**
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* Inserts element at current put position, advances, and signals.
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* Call only when holding lock.
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*/
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private void insert(E x) {
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items[putIndex] = x;
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putIndex = inc(putIndex);
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++count;
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notEmpty.signal();
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}
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/**
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* Extracts element at current take position, advances, and signals.
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* Call only when holding lock.
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*/
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private E extract() {
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final E[] items = this.items;
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E x = items[takeIndex];
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items[takeIndex] = null;
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takeIndex = inc(takeIndex);
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--count;
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notFull.signal();
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return x;
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}
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/**
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* Utility for remove and iterator.remove: Delete item at position i.
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* Call only when holding lock.
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*/
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void removeAt(int i) {
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final E[] items = this.items;
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// if removing front item, just advance
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if (i == takeIndex) {
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items[takeIndex] = null;
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takeIndex = inc(takeIndex);
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} else {
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// slide over all others up through putIndex.
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for (;;) {
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int nexti = inc(i);
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if (nexti != putIndex) {
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items[i] = items[nexti];
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i = nexti;
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} else {
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items[i] = null;
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putIndex = i;
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break;
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}
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}
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}
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--count;
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notFull.signal();
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}
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/**
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* Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
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* capacity and default access policy.
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*
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* @param capacity the capacity of this queue
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* @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
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*/
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public ArrayBlockingQueue(int capacity) {
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this(capacity, false);
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}
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/**
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* Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
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* capacity and the specified access policy.
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*
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* @param capacity the capacity of this queue
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* @param fair if <tt>true</tt> then queue accesses for threads blocked
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* on insertion or removal, are processed in FIFO order;
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* if <tt>false</tt> the access order is unspecified.
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* @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
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*/
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public ArrayBlockingQueue(int capacity, boolean fair) {
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if (capacity <= 0)
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throw new IllegalArgumentException();
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this.items = (E[]) new Object[capacity];
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lock = new ReentrantLock(fair);
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notEmpty = lock.newCondition();
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notFull = lock.newCondition();
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}
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/**
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* Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
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* capacity, the specified access policy and initially containing the
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* elements of the given collection,
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* added in traversal order of the collection's iterator.
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*
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* @param capacity the capacity of this queue
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* @param fair if <tt>true</tt> then queue accesses for threads blocked
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* on insertion or removal, are processed in FIFO order;
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* if <tt>false</tt> the access order is unspecified.
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* @param c the collection of elements to initially contain
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* @throws IllegalArgumentException if <tt>capacity</tt> is less than
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* <tt>c.size()</tt>, or less than 1.
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* @throws NullPointerException if the specified collection or any
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* of its elements are null
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*/
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public ArrayBlockingQueue(int capacity, boolean fair,
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Collection<? extends E> c) {
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this(capacity, fair);
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if (capacity < c.size())
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throw new IllegalArgumentException();
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for (Iterator<? extends E> it = c.iterator(); it.hasNext();)
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add(it.next());
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}
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/**
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* Inserts the specified element at the tail of this queue if it is
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* possible to do so immediately without exceeding the queue's capacity,
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* returning <tt>true</tt> upon success and throwing an
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* <tt>IllegalStateException</tt> if this queue is full.
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*
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* @param e the element to add
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* @return <tt>true</tt> (as specified by {@link Collection#add})
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* @throws IllegalStateException if this queue is full
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* @throws NullPointerException if the specified element is null
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*/
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public boolean add(E e) {
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return super.add(e);
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}
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/**
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* Inserts the specified element at the tail of this queue if it is
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* possible to do so immediately without exceeding the queue's capacity,
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* returning <tt>true</tt> upon success and <tt>false</tt> if this queue
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* is full. This method is generally preferable to method {@link #add},
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* which can fail to insert an element only by throwing an exception.
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*
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* @throws NullPointerException if the specified element is null
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*/
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public boolean offer(E e) {
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if (e == null) throw new NullPointerException();
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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if (count == items.length)
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return false;
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else {
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insert(e);
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return true;
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}
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} finally {
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lock.unlock();
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}
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}
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/**
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* Inserts the specified element at the tail of this queue, waiting
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* for space to become available if the queue is full.
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*
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* @throws InterruptedException {@inheritDoc}
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* @throws NullPointerException {@inheritDoc}
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*/
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public void put(E e) throws InterruptedException {
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if (e == null) throw new NullPointerException();
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final E[] items = this.items;
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final ReentrantLock lock = this.lock;
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lock.lockInterruptibly();
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try {
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try {
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while (count == items.length)
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notFull.await();
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} catch (InterruptedException ie) {
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notFull.signal(); // propagate to non-interrupted thread
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throw ie;
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}
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insert(e);
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} finally {
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lock.unlock();
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}
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}
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/**
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* Inserts the specified element at the tail of this queue, waiting
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* up to the specified wait time for space to become available if
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* the queue is full.
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*
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* @throws InterruptedException {@inheritDoc}
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* @throws NullPointerException {@inheritDoc}
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*/
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public boolean offer(E e, long timeout, TimeUnit unit)
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throws InterruptedException {
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if (e == null) throw new NullPointerException();
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long nanos = unit.toNanos(timeout);
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final ReentrantLock lock = this.lock;
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lock.lockInterruptibly();
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try {
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for (;;) {
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if (count != items.length) {
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insert(e);
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return true;
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}
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if (nanos <= 0)
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return false;
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try {
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nanos = notFull.awaitNanos(nanos);
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} catch (InterruptedException ie) {
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notFull.signal(); // propagate to non-interrupted thread
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throw ie;
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}
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}
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} finally {
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lock.unlock();
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}
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}
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public E poll() {
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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if (count == 0)
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return null;
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E x = extract();
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return x;
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} finally {
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lock.unlock();
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}
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}
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public E take() throws InterruptedException {
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final ReentrantLock lock = this.lock;
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lock.lockInterruptibly();
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try {
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try {
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while (count == 0)
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notEmpty.await();
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} catch (InterruptedException ie) {
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notEmpty.signal(); // propagate to non-interrupted thread
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throw ie;
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}
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E x = extract();
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return x;
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} finally {
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lock.unlock();
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}
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}
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public E poll(long timeout, TimeUnit unit) throws InterruptedException {
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long nanos = unit.toNanos(timeout);
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final ReentrantLock lock = this.lock;
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lock.lockInterruptibly();
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try {
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for (;;) {
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if (count != 0) {
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E x = extract();
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return x;
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}
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if (nanos <= 0)
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return null;
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try {
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nanos = notEmpty.awaitNanos(nanos);
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} catch (InterruptedException ie) {
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notEmpty.signal(); // propagate to non-interrupted thread
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throw ie;
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}
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}
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} finally {
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lock.unlock();
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}
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}
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public E peek() {
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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return (count == 0) ? null : items[takeIndex];
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} finally {
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lock.unlock();
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}
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}
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// this doc comment is overridden to remove the reference to collections
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// greater in size than Integer.MAX_VALUE
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/**
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* Returns the number of elements in this queue.
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*
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* @return the number of elements in this queue
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*/
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public int size() {
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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return count;
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} finally {
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lock.unlock();
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}
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}
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// this doc comment is a modified copy of the inherited doc comment,
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// without the reference to unlimited queues.
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/**
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* Returns the number of additional elements that this queue can ideally
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* (in the absence of memory or resource constraints) accept without
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* blocking. This is always equal to the initial capacity of this queue
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* less the current <tt>size</tt> of this queue.
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*
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* <p>Note that you <em>cannot</em> always tell if an attempt to insert
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* an element will succeed by inspecting <tt>remainingCapacity</tt>
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* because it may be the case that another thread is about to
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* insert or remove an element.
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*/
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public int remainingCapacity() {
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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return items.length - count;
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} finally {
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lock.unlock();
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}
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}
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/**
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* Removes a single instance of the specified element from this queue,
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* if it is present. More formally, removes an element <tt>e</tt> such
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* that <tt>o.equals(e)</tt>, if this queue contains one or more such
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* elements.
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* Returns <tt>true</tt> if this queue contained the specified element
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* (or equivalently, if this queue changed as a result of the call).
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*
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* @param o element to be removed from this queue, if present
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* @return <tt>true</tt> if this queue changed as a result of the call
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*/
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public boolean remove(Object o) {
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if (o == null) return false;
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final E[] items = this.items;
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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int i = takeIndex;
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int k = 0;
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for (;;) {
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if (k++ >= count)
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return false;
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if (o.equals(items[i])) {
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removeAt(i);
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return true;
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}
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i = inc(i);
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}
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} finally {
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lock.unlock();
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}
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}
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/**
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* Returns <tt>true</tt> if this queue contains the specified element.
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* More formally, returns <tt>true</tt> if and only if this queue contains
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* at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
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*
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* @param o object to be checked for containment in this queue
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* @return <tt>true</tt> if this queue contains the specified element
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*/
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public boolean contains(Object o) {
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if (o == null) return false;
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final E[] items = this.items;
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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int i = takeIndex;
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int k = 0;
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while (k++ < count) {
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if (o.equals(items[i]))
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return true;
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i = inc(i);
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}
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return false;
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} finally {
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lock.unlock();
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}
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}
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/**
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* Returns an array containing all of the elements in this queue, in
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* proper sequence.
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*
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* <p>The returned array will be "safe" in that no references to it are
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* maintained by this queue. (In other words, this method must allocate
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* a new array). The caller is thus free to modify the returned array.
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*
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* <p>This method acts as bridge between array-based and collection-based
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* APIs.
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*
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* @return an array containing all of the elements in this queue
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*/
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public Object[] toArray() {
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final E[] items = this.items;
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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Object[] a = new Object[count];
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int k = 0;
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int i = takeIndex;
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while (k < count) {
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a[k++] = items[i];
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i = inc(i);
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}
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return a;
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} finally {
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lock.unlock();
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}
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}
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/**
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* Returns an array containing all of the elements in this queue, in
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* proper sequence; the runtime type of the returned array is that of
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* the specified array. If the queue fits in the specified array, it
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* is returned therein. Otherwise, a new array is allocated with the
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* runtime type of the specified array and the size of this queue.
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*
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* <p>If this queue fits in the specified array with room to spare
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* (i.e., the array has more elements than this queue), the element in
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* the array immediately following the end of the queue is set to
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* <tt>null</tt>.
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*
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* <p>Like the {@link #toArray()} method, this method acts as bridge between
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* array-based and collection-based APIs. Further, this method allows
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* precise control over the runtime type of the output array, and may,
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* under certain circumstances, be used to save allocation costs.
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*
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* <p>Suppose <tt>x</tt> is a queue known to contain only strings.
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* The following code can be used to dump the queue into a newly
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* allocated array of <tt>String</tt>:
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*
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* <pre>
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* String[] y = x.toArray(new String[0]);</pre>
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*
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* Note that <tt>toArray(new Object[0])</tt> is identical in function to
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* <tt>toArray()</tt>.
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*
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* @param a the array into which the elements of the queue are to
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* be stored, if it is big enough; otherwise, a new array of the
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* same runtime type is allocated for this purpose
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* @return an array containing all of the elements in this queue
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* @throws ArrayStoreException if the runtime type of the specified array
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* is not a supertype of the runtime type of every element in
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* this queue
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* @throws NullPointerException if the specified array is null
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*/
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public <T> T[] toArray(T[] a) {
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final E[] items = this.items;
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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if (a.length < count)
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a = (T[])java.lang.reflect.Array.newInstance(
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a.getClass().getComponentType(),
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count
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);
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int k = 0;
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int i = takeIndex;
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while (k < count) {
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a[k++] = (T)items[i];
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i = inc(i);
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}
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if (a.length > count)
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a[count] = null;
|
|
return a;
|
|
} finally {
|
|
lock.unlock();
|
|
}
|
|
}
|
|
|
|
public String toString() {
|
|
final ReentrantLock lock = this.lock;
|
|
lock.lock();
|
|
try {
|
|
return super.toString();
|
|
} finally {
|
|
lock.unlock();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Atomically removes all of the elements from this queue.
|
|
* The queue will be empty after this call returns.
|
|
*/
|
|
public void clear() {
|
|
final E[] items = this.items;
|
|
final ReentrantLock lock = this.lock;
|
|
lock.lock();
|
|
try {
|
|
int i = takeIndex;
|
|
int k = count;
|
|
while (k-- > 0) {
|
|
items[i] = null;
|
|
i = inc(i);
|
|
}
|
|
count = 0;
|
|
putIndex = 0;
|
|
takeIndex = 0;
|
|
notFull.signalAll();
|
|
} finally {
|
|
lock.unlock();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* @throws UnsupportedOperationException {@inheritDoc}
|
|
* @throws ClassCastException {@inheritDoc}
|
|
* @throws NullPointerException {@inheritDoc}
|
|
* @throws IllegalArgumentException {@inheritDoc}
|
|
*/
|
|
public int drainTo(Collection<? super E> c) {
|
|
if (c == null)
|
|
throw new NullPointerException();
|
|
if (c == this)
|
|
throw new IllegalArgumentException();
|
|
final E[] items = this.items;
|
|
final ReentrantLock lock = this.lock;
|
|
lock.lock();
|
|
try {
|
|
int i = takeIndex;
|
|
int n = 0;
|
|
int max = count;
|
|
while (n < max) {
|
|
c.add(items[i]);
|
|
items[i] = null;
|
|
i = inc(i);
|
|
++n;
|
|
}
|
|
if (n > 0) {
|
|
count = 0;
|
|
putIndex = 0;
|
|
takeIndex = 0;
|
|
notFull.signalAll();
|
|
}
|
|
return n;
|
|
} finally {
|
|
lock.unlock();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* @throws UnsupportedOperationException {@inheritDoc}
|
|
* @throws ClassCastException {@inheritDoc}
|
|
* @throws NullPointerException {@inheritDoc}
|
|
* @throws IllegalArgumentException {@inheritDoc}
|
|
*/
|
|
public int drainTo(Collection<? super E> c, int maxElements) {
|
|
if (c == null)
|
|
throw new NullPointerException();
|
|
if (c == this)
|
|
throw new IllegalArgumentException();
|
|
if (maxElements <= 0)
|
|
return 0;
|
|
final E[] items = this.items;
|
|
final ReentrantLock lock = this.lock;
|
|
lock.lock();
|
|
try {
|
|
int i = takeIndex;
|
|
int n = 0;
|
|
int sz = count;
|
|
int max = (maxElements < count)? maxElements : count;
|
|
while (n < max) {
|
|
c.add(items[i]);
|
|
items[i] = null;
|
|
i = inc(i);
|
|
++n;
|
|
}
|
|
if (n > 0) {
|
|
count -= n;
|
|
takeIndex = i;
|
|
notFull.signalAll();
|
|
}
|
|
return n;
|
|
} finally {
|
|
lock.unlock();
|
|
}
|
|
}
|
|
|
|
|
|
/**
|
|
* Returns an iterator over the elements in this queue in proper sequence.
|
|
* The returned <tt>Iterator</tt> is a "weakly consistent" iterator that
|
|
* will never throw {@link ConcurrentModificationException},
|
|
* and guarantees to traverse elements as they existed upon
|
|
* construction of the iterator, and may (but is not guaranteed to)
|
|
* reflect any modifications subsequent to construction.
|
|
*
|
|
* @return an iterator over the elements in this queue in proper sequence
|
|
*/
|
|
public Iterator<E> iterator() {
|
|
final ReentrantLock lock = this.lock;
|
|
lock.lock();
|
|
try {
|
|
return new Itr();
|
|
} finally {
|
|
lock.unlock();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Iterator for ArrayBlockingQueue
|
|
*/
|
|
private class Itr implements Iterator<E> {
|
|
/**
|
|
* Index of element to be returned by next,
|
|
* or a negative number if no such.
|
|
*/
|
|
private int nextIndex;
|
|
|
|
/**
|
|
* nextItem holds on to item fields because once we claim
|
|
* that an element exists in hasNext(), we must return it in
|
|
* the following next() call even if it was in the process of
|
|
* being removed when hasNext() was called.
|
|
*/
|
|
private E nextItem;
|
|
|
|
/**
|
|
* Index of element returned by most recent call to next.
|
|
* Reset to -1 if this element is deleted by a call to remove.
|
|
*/
|
|
private int lastRet;
|
|
|
|
Itr() {
|
|
lastRet = -1;
|
|
if (count == 0)
|
|
nextIndex = -1;
|
|
else {
|
|
nextIndex = takeIndex;
|
|
nextItem = items[takeIndex];
|
|
}
|
|
}
|
|
|
|
public boolean hasNext() {
|
|
/*
|
|
* No sync. We can return true by mistake here
|
|
* only if this iterator passed across threads,
|
|
* which we don't support anyway.
|
|
*/
|
|
return nextIndex >= 0;
|
|
}
|
|
|
|
/**
|
|
* Checks whether nextIndex is valid; if so setting nextItem.
|
|
* Stops iterator when either hits putIndex or sees null item.
|
|
*/
|
|
private void checkNext() {
|
|
if (nextIndex == putIndex) {
|
|
nextIndex = -1;
|
|
nextItem = null;
|
|
} else {
|
|
nextItem = items[nextIndex];
|
|
if (nextItem == null)
|
|
nextIndex = -1;
|
|
}
|
|
}
|
|
|
|
public E next() {
|
|
final ReentrantLock lock = ArrayBlockingQueue.this.lock;
|
|
lock.lock();
|
|
try {
|
|
if (nextIndex < 0)
|
|
throw new NoSuchElementException();
|
|
lastRet = nextIndex;
|
|
E x = nextItem;
|
|
nextIndex = inc(nextIndex);
|
|
checkNext();
|
|
return x;
|
|
} finally {
|
|
lock.unlock();
|
|
}
|
|
}
|
|
|
|
public void remove() {
|
|
final ReentrantLock lock = ArrayBlockingQueue.this.lock;
|
|
lock.lock();
|
|
try {
|
|
int i = lastRet;
|
|
if (i == -1)
|
|
throw new IllegalStateException();
|
|
lastRet = -1;
|
|
|
|
int ti = takeIndex;
|
|
removeAt(i);
|
|
// back up cursor (reset to front if was first element)
|
|
nextIndex = (i == ti) ? takeIndex : i;
|
|
checkNext();
|
|
} finally {
|
|
lock.unlock();
|
|
}
|
|
}
|
|
}
|
|
}
|