java.util.concurrent
class PriorityBlockingQueue

java.lang.Object java.util.AbstractCollection java.util.AbstractQueue java.util.concurrent.PriorityBlockingQueue

All Implemented Interfaces:

Serializable, Collection, Queue, BlockingQueue

public class PriorityBlockingQueue
extends AbstractQueue
implements BlockingQueue, Serializable

An unbounded {@linkplain BlockingQueue blocking queue} that uses the same ordering rules as class {@link PriorityQueue} and supplies blocking retrieval operations. While this queue is logically unbounded, attempted additions may fail due to resource exhaustion (causing OutOfMemoryError). This class does not permit null elements. A priority queue relying on {@linkplain Comparable natural ordering} also does not permit insertion of non-comparable objects (doing so results in ClassCastException).

This class and its iterator implement all of the optional methods of the {@link Collection} and {@link Iterator} interfaces. The Iterator provided in method {@link #iterator()} is not guaranteed to traverse the elements of the PriorityBlockingQueue in any particular order. If you need ordered traversal, consider using Arrays.sort(pq.toArray()). Also, method drainTo can be used to remove some or all elements in priority order and place them in another collection.

Operations on this class make no guarantees about the ordering of elements with equal priority. If you need to enforce an ordering, you can define custom classes or comparators that use a secondary key to break ties in primary priority values. For example, here is a class that applies first-in-first-out tie-breaking to comparable elements. To use it, you would insert a new FIFOEntry(anEntry) instead of a plain entry object.

 class FIFOEntry<E extends Comparable<? super E>>
     implements Comparable<FIFOEntry<E>> {
   final static AtomicLong seq = new AtomicLong();
   final long seqNum;
   final E entry;
   public FIFOEntry(E entry) {
     seqNum = seq.getAndIncrement();
     this.entry = entry;
   }
   public E getEntry() { return entry; }
   public int compareTo(FIFOEntry<E> other) {
     int res = entry.compareTo(other.entry);
     if (res == 0 && other.entry != this.entry)
       res = (seqNum < other.seqNum ? -1 : 1);
     return res;
   }
 }

This class is a member of the Java Collections Framework.

PriorityBlockingQueue

public PriorityBlockingQueue()

Creates a PriorityBlockingQueue with the default initial capacity (11) that orders its elements according to their {@linkplain Comparable natural ordering}.

PriorityBlockingQueue

public PriorityBlockingQueue(Collection c)

Creates a PriorityBlockingQueue containing the elements in the specified collection. If the specified collection is a {@link SortedSet} or a {@link PriorityQueue}, this priority queue will be ordered according to the same ordering. Otherwise, this priority queue will be ordered according to the {@linkplain Comparable natural ordering} of its elements.

Parameters:

c - the collection whose elements are to be placed into this priority queue

PriorityBlockingQueue

public PriorityBlockingQueue(int initialCapacity)

Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to their {@linkplain Comparable natural ordering}.

Parameters:

initialCapacity - the initial capacity for this priority queue

PriorityBlockingQueue

public PriorityBlockingQueue(int initialCapacity,
                             Comparator comparator)

Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to the specified comparator.

Parameters:

initialCapacity - the initial capacity for this priority queue

comparator - the comparator that will be used to order this priority queue. If {@code null}, the {@linkplain Comparable natural ordering} of the elements will be used.

add

public boolean add(Object e)

Inserts the specified element into this priority queue.

Overrides:

add in class AbstractQueue

Parameters:

e - the element to add

Returns:

true (as specified by {@link Collection#add})

clear

public void clear()

Atomically removes all of the elements from this queue. The queue will be empty after this call returns.

Overrides:

clear in class AbstractQueue

comparator

public Comparator comparator()

Returns the comparator used to order the elements in this queue, or null if this queue uses the {@linkplain Comparable natural ordering} of its elements.

Returns:

the comparator used to order the elements in this queue, or null if this queue uses the natural ordering of its elements

contains

public boolean contains(Object o)

Returns {@code true} if this queue contains the specified element. More formally, returns {@code true} if and only if this queue contains at least one element {@code e} such that {@code o.equals(e)}.

Overrides:

contains in class AbstractCollection

Parameters:

o - object to be checked for containment in this queue

Returns:

true if this queue contains the specified element

drainTo

public int drainTo(Collection c)

Parameters:

c

drainTo

public int drainTo(Collection c,
                   int maxElements)

Parameters:

c

maxElements

iterator

public Iterator iterator()

Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order. The returned Iterator 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.

Overrides:

iterator in class AbstractCollection

Returns:

an iterator over the elements in this queue

offer

public boolean offer(Object e)

Inserts the specified element into this priority queue.

Parameters:

e - the element to add

Returns:

true (as specified by {@link Queue#offer})

offer

public boolean offer(Object e,
                     long timeout,
                     TimeUnit unit)

Inserts the specified element into this priority queue. As the queue is unbounded this method will never block.

Parameters:

e - the element to add

timeout - This parameter is ignored as the method never blocks

unit - This parameter is ignored as the method never blocks

Returns:

true

peek

public Object peek()

poll

public Object poll()

poll

public Object poll(long timeout,
                   TimeUnit unit)
            throws InterruptedException

Parameters:

timeout

unit

Throws:

InterruptedException

put

public void put(Object e)

Inserts the specified element into this priority queue. As the queue is unbounded this method will never block.

Parameters:

e - the element to add

remainingCapacity

public int remainingCapacity()

Always returns Integer.MAX_VALUE because a PriorityBlockingQueue is not capacity constrained.

Returns:

Integer.MAX_VALUE

remove

public boolean remove(Object o)

Removes a single instance of the specified element from this queue, if it is present. More formally, removes an element {@code e} such that {@code o.equals(e)}, if this queue contains one or more such elements. Returns {@code true} if and only if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).

Overrides:

remove in class AbstractCollection

Parameters:

o - element to be removed from this queue, if present

Returns:

true if this queue changed as a result of the call

size

public int size()

Overrides:

size in class AbstractCollection

take

public Object take()
            throws InterruptedException

Throws:

InterruptedException

toArray

public Object[] toArray()

Returns an array containing all of the elements in this queue. The returned array elements are in no particular order.

The returned array will be "safe" in that no references to it are maintained by this queue. (In other words, this method must allocate a new array). The caller is thus free to modify the returned array.

This method acts as bridge between array-based and collection-based APIs.

Overrides:

toArray in class AbstractCollection

Returns:

an array containing all of the elements in this queue

toArray

public Object[] toArray(Object[] a)

Returns an array containing all of the elements in this queue; the runtime type of the returned array is that of the specified array. The returned array elements are in no particular order. If the queue fits in the specified array, it is returned therein. Otherwise, a new array is allocated with the runtime type of the specified array and the size of this queue.

If this queue fits in the specified array with room to spare (i.e., the array has more elements than this queue), the element in the array immediately following the end of the queue is set to null.

Like the {@link #toArray()} method, this method acts as bridge between array-based and collection-based APIs. Further, this method allows precise control over the runtime type of the output array, and may, under certain circumstances, be used to save allocation costs.

Suppose x is a queue known to contain only strings. The following code can be used to dump the queue into a newly allocated array of String:

     String[] y = x.toArray(new String[0]);

Note that toArray(new Object[0]) is identical in function to toArray().

Overrides:

toArray in class AbstractCollection

Parameters:

a - the array into which the elements of the queue are to be stored, if it is big enough; otherwise, a new array of the same runtime type is allocated for this purpose

Returns:

an array containing all of the elements in this queue

toString

public String toString()

Overrides:

toString in class AbstractCollection