java.util.concurrent

Class PriorityBlockingQueue<E>

Implemented Interfaces:
BlockingQueue<E>, Collection<E>, Iterable<E>, Queue<E>, Serializable

public class PriorityBlockingQueue<E>
extends AbstractQueue<E>
implements BlockingQueue<E>, Serializable

An unbounded blocking queue that uses the same ordering rules as class 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 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 Collection and Iterator interfaces. The Iterator provided in method 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 ../../../../technotes/guides/collections/index.html"> Java Collections Framework.

Since:
1.5
See Also:
Serialized Form

Constructor Summary

PriorityBlockingQueue()
Creates a PriorityBlockingQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
PriorityBlockingQueue(E> c)
Creates a PriorityBlockingQueue containing the elements in the specified collection.
PriorityBlockingQueue(int initialCapacity)
Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to their natural ordering.
PriorityBlockingQueue(int initialCapacity, E> comparator)
Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to the specified comparator.

Method Summary

T[] toArray(T[] 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.
boolean
add(E e)
Inserts the specified element into this priority queue.
void
clear()
Atomically removes all of the elements from this queue.
boolean
contains(Object o)
Returns true if this queue contains the specified element.
int
drainTo(E> c)
int
drainTo(E> c, int maxElements)
Iterator
iterator()
Returns an iterator over the elements in this queue.
boolean
offer(E e)
Inserts the specified element into this priority queue.
boolean
offer(E e, long timeout, TimeUnit unit)
Inserts the specified element into this priority queue.
E
peek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
E
poll()
Retrieves and removes the head of this queue, or returns null if this queue is empty.
E
poll(long timeout, TimeUnit unit)
void
put(E e)
Inserts the specified element into this priority queue.
int
remainingCapacity()
Always returns Integer.MAX_VALUE because a PriorityBlockingQueue is not capacity constrained.
boolean
remove(Object o)
Removes a single instance of the specified element from this queue, if it is present.
int
size()
Comparator
super E> comparator()
Returns the comparator used to order the elements in this queue, or null if this queue uses the natural ordering of its elements.
E
take()
Object[]
toArray()
Returns an array containing all of the elements in this queue.
String
toString()
Convert this Object to a human-readable String.

Methods inherited from class java.util.AbstractQueue<E>

add, addAll, clear, element, remove

Methods inherited from class java.util.AbstractCollection<E>

T[] toArray, add, addAll, clear, contains, containsAll, isEmpty, iterator, remove, removeAll, retainAll, size, toArray, toString

Methods inherited from class java.lang.Object

clone, equals, extends Object> getClass, finalize, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Details

PriorityBlockingQueue

public PriorityBlockingQueue()
Creates a PriorityBlockingQueue with the default initial capacity (11) that orders its elements according to their natural ordering.

PriorityBlockingQueue

public PriorityBlockingQueue(E> c)
Creates a PriorityBlockingQueue containing the elements in the specified collection. If the specified collection is a SortedSet or a PriorityQueue, this priority queue will be ordered according to the same ordering. Otherwise, this priority queue will be ordered according to the natural ordering of its elements.
Parameters:
c - the collection whose elements are to be placed into this priority queue
Throws:
ClassCastException - if elements of the specified collection cannot be compared to one another according to the priority queue's ordering
NullPointerException - if the specified collection or any of its elements are null

PriorityBlockingQueue

public PriorityBlockingQueue(int initialCapacity)
Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to their natural ordering.
Parameters:
initialCapacity - the initial capacity for this priority queue
Throws:
IllegalArgumentException - if initialCapacity is less than 1

PriorityBlockingQueue

public PriorityBlockingQueue(int initialCapacity,
                             E> 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 null, the natural ordering of the elements will be used.
Throws:
IllegalArgumentException - if initialCapacity is less than 1

Method Details

T[] toArray

public  T[] toArray(T[] 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 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().
Specified by:
T[] toArray in interface Collection<E>
Overrides:
T[] toArray in interface AbstractCollection<E>
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
Throws:
ArrayStoreException - if the runtime type of the specified array is not a supertype of the runtime type of every element in this queue
NullPointerException - if the specified array is null

add

public boolean add(E e)
Inserts the specified element into this priority queue.
Specified by:
add in interface BlockingQueue<E>
add in interface Queue<E>
add in interface Collection<E>
Overrides:
add in interface AbstractQueue<E>
Parameters:
e - the element to add
Returns:
true (as specified by Collection.add(E))
Throws:
ClassCastException - if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering
NullPointerException - if the specified element is null

clear

public void clear()
Atomically removes all of the elements from this queue. The queue will be empty after this call returns.
Specified by:
clear in interface Collection<E>
Overrides:
clear in interface AbstractQueue<E>

contains

public boolean contains(Object o)
Returns true if this queue contains the specified element. More formally, returns true if and only if this queue contains at least one element e such that o.equals(e).
Specified by:
contains in interface BlockingQueue<E>
contains in interface Collection<E>
Overrides:
contains in interface AbstractCollection<E>
Parameters:
o - object to be checked for containment in this queue
Returns:
true if this queue contains the specified element

drainTo

public int drainTo(E> c)
Specified by:
drainTo in interface BlockingQueue<E>

drainTo

public int drainTo(E> c,
                   int maxElements)
Specified by:
drainTo in interface BlockingQueue<E>

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 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.
Specified by:
iterator in interface Collection<E>
iterator in interface Iterable<E>
Overrides:
iterator in interface AbstractCollection<E>
Returns:
an iterator over the elements in this queue

offer

public boolean offer(E e)
Inserts the specified element into this priority queue.
Specified by:
offer in interface BlockingQueue<E>
offer in interface Queue<E>
Parameters:
e - the element to add
Returns:
true (as specified by Queue.offer(E))
Throws:
ClassCastException - if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering
NullPointerException - if the specified element is null

offer

public boolean offer(E e,
                     long timeout,
                     TimeUnit unit)
Inserts the specified element into this priority queue. As the queue is unbounded this method will never block.
Specified by:
offer in interface BlockingQueue<E>
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
Throws:
ClassCastException - if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering
NullPointerException - if the specified element is null

peek

public E peek()
Retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
Specified by:
peek in interface Queue<E>
Returns:
the head of this queue, or null if this queue is empty

poll

public E poll()
Retrieves and removes the head of this queue, or returns null if this queue is empty.
Specified by:
poll in interface Queue<E>
Returns:
the head of this queue, or null if this queue is empty

poll

public E poll(long timeout,
              TimeUnit unit)
            throws InterruptedException
Specified by:
poll in interface BlockingQueue<E>

put

public void put(E e)
Inserts the specified element into this priority queue. As the queue is unbounded this method will never block.
Specified by:
put in interface BlockingQueue<E>
Parameters:
e - the element to add
Throws:
ClassCastException - if the specified element cannot be compared with elements currently in the priority queue according to the priority queue's ordering
NullPointerException - if the specified element is null

remainingCapacity

public int remainingCapacity()
Always returns Integer.MAX_VALUE because a PriorityBlockingQueue is not capacity constrained.
Specified by:
remainingCapacity in interface BlockingQueue<E>
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 e such that o.equals(e), if this queue contains one or more such elements. Returns true if and only if this queue contained the specified element (or equivalently, if this queue changed as a result of the call).
Specified by:
remove in interface BlockingQueue<E>
remove in interface Collection<E>
Overrides:
remove in interface AbstractCollection<E>
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()
Specified by:
size in interface Collection<E>
Overrides:
size in interface AbstractCollection<E>

super E> comparator

public Comparatorsuper E> comparator()
Returns the comparator used to order the elements in this queue, or null if this queue uses the 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

take

public E take()
            throws InterruptedException
Specified by:
take in interface BlockingQueue<E>

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.

Specified by:
toArray in interface Collection<E>
Overrides:
toArray in interface AbstractCollection<E>
Returns:
an array containing all of the elements in this queue

toString

public String toString()
Convert this Object to a human-readable String. There are no limits placed on how long this String should be or what it should contain. We suggest you make it as intuitive as possible to be able to place it into System.out.println() and such.

It is typical, but not required, to ensure that this method never completes abruptly with a RuntimeException.

This method will be called when performing string concatenation with this object. If the result is null, string concatenation will instead use "null".

The default implementation returns getClass().getName() + "@" + Integer.toHexString(hashCode()).

Overrides:
toString in interface AbstractCollection<E>
Returns:
the String representing this Object, which may be null