Module java.base
Package java.util.concurrent

Class PriorityBlockingQueue<E>

java.lang.Object
java.util.AbstractCollection<E>
java.util.AbstractQueue<E>
java.util.concurrent.PriorityBlockingQueue<E>
Type Parameters:
E - the type of elements held in this queue
All Implemented Interfaces:
Serializable, Iterable<E>, Collection<E>, BlockingQueue<E>, Queue<E>
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() and the Spliterator provided in method spliterator() are 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>> {
   static final 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.

Since:
1.5
See Also:

  • Constructor Summary

    Constructors
    Constructor
    Description
    PriorityBlockingQueue()
    Creates a PriorityBlockingQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
    PriorityBlockingQueue(int initialCapacity)
    Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to their natural ordering.
    PriorityBlockingQueue(int initialCapacity, Comparator<? super E> comparator)
    Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to the specified comparator.
    PriorityBlockingQueue(Collection<? extends E> c)
    Creates a PriorityBlockingQueue containing the elements in the specified collection.

  • Method Summary

    Modifier and Type
    Method
    Description
    boolean
    add(E e)
    Inserts the specified element into this priority queue.
    void
    clear()
    Atomically removes all of the elements from this queue.
    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.
    boolean
    contains(Object o)
    Returns true if this queue contains the specified element.
    int
    drainTo(Collection<? super E> c)
    Removes all available elements from this queue and adds them to the given collection.
    int
    drainTo(Collection<? super E> c, int maxElements)
    Removes at most the given number of available elements from this queue and adds them to the given collection.
    void
    forEach(Consumer<? super E> action)
    Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception.
    Iterator<E>
    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)
    Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
    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.
    boolean
    removeAll(Collection<?> c)
    Removes all of this collection's elements that are also contained in the specified collection (optional operation).
    boolean
    removeIf(Predicate<? super E> filter)
    Removes all of the elements of this collection that satisfy the given predicate.
    boolean
    retainAll(Collection<?> c)
    Retains only the elements in this collection that are contained in the specified collection (optional operation).
    int
    size()
    Returns the number of elements in this collection.
    Spliterator<E>
    spliterator()
    Returns a Spliterator over the elements in this queue.
    E
    take()
    Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
    Object[]
    toArray()
    Returns an array containing all of the elements in this queue.
    <T> 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.

    Methods declared in class java.util.AbstractQueue

    addAll, element, remove

    Methods declared in class java.util.AbstractCollection

    containsAll, isEmpty, toString

    Methods declared in class java.lang.Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

    Methods declared in interface java.util.Collection

    addAll, containsAll, equals, hashCode, isEmpty, parallelStream, stream, toArray

    Methods declared in interface java.util.Queue

    element, remove

  • 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(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, Comparator<? super 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

    • PriorityBlockingQueue

      public PriorityBlockingQueue(Collection<? extends E> c)
      Creates a PriorityBlockingQueue containing the elements in the specified collection. If the specified collection is a SortedSet or a PriorityBlockingQueue, 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

  • Method Details

    • add

      public boolean add(E e)
      Inserts the specified element into this priority queue.
      Specified by:
      add in interface BlockingQueue<E>
      Specified by:
      add in interface Collection<E>
      Specified by:
      add in interface Queue<E>
      Overrides:
      add in class 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

    • offer

      public boolean offer(E e)
      Inserts the specified element into this priority queue. As the queue is unbounded, this method will never return false.
      Specified by:
      offer in interface BlockingQueue<E>
      Specified by:
      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

    • 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

    • 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 or return false.
      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 (as specified by BlockingQueue.offer)
      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

    • poll

      public E poll()
      Description copied from interface: Queue
      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

    • take

      public E take() throws InterruptedException
      Description copied from interface: BlockingQueue
      Retrieves and removes the head of this queue, waiting if necessary until an element becomes available.
      Specified by:
      take in interface BlockingQueue<E>
      Returns:
      the head of this queue
      Throws:
      InterruptedException - if interrupted while waiting

    • poll

      public E poll(long timeout, TimeUnit unit) throws InterruptedException
      Description copied from interface: BlockingQueue
      Retrieves and removes the head of this queue, waiting up to the specified wait time if necessary for an element to become available.
      Specified by:
      poll in interface BlockingQueue<E>
      Parameters:
      timeout - how long to wait before giving up, in units of unit
      unit - a TimeUnit determining how to interpret the timeout parameter
      Returns:
      the head of this queue, or null if the specified waiting time elapses before an element is available
      Throws:
      InterruptedException - if interrupted while waiting

    • peek

      public E peek()
      Description copied from interface: Queue
      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

    • comparator

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

    • size

      public int size()
      Description copied from interface: Collection
      Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
      Specified by:
      size in interface Collection<E>
      Returns:
      the number of elements in this collection

    • 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 always

    • 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>
      Specified by:
      remove in interface Collection<E>
      Overrides:
      remove in class 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

    • 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>
      Specified by:
      contains in interface Collection<E>
      Overrides:
      contains in class 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(Collection<? super E> c)
      Description copied from interface: BlockingQueue
      Removes all available elements from this queue and adds them to the given collection. This operation may be more efficient than repeatedly polling this queue. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
      Specified by:
      drainTo in interface BlockingQueue<E>
      Parameters:
      c - the collection to transfer elements into
      Returns:
      the number of elements transferred
      Throws:
      UnsupportedOperationException - if addition of elements is not supported by the specified collection
      ClassCastException - if the class of an element of this queue prevents it from being added to the specified collection
      NullPointerException - if the specified collection is null
      IllegalArgumentException - if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection

    • drainTo

      public int drainTo(Collection<? super E> c, int maxElements)
      Description copied from interface: BlockingQueue
      Removes at most the given number of available elements from this queue and adds them to the given collection. A failure encountered while attempting to add elements to collection c may result in elements being in neither, either or both collections when the associated exception is thrown. Attempts to drain a queue to itself result in IllegalArgumentException. Further, the behavior of this operation is undefined if the specified collection is modified while the operation is in progress.
      Specified by:
      drainTo in interface BlockingQueue<E>
      Parameters:
      c - the collection to transfer elements into
      maxElements - the maximum number of elements to transfer
      Returns:
      the number of elements transferred
      Throws:
      UnsupportedOperationException - if addition of elements is not supported by the specified collection
      ClassCastException - if the class of an element of this queue prevents it from being added to the specified collection
      NullPointerException - if the specified collection is null
      IllegalArgumentException - if the specified collection is this queue, or some property of an element of this queue prevents it from being added to the specified collection

    • 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 class AbstractQueue<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 class AbstractCollection<E>
      Returns:
      an array containing all of the elements in this queue

    • toArray

      public <T> 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:
      toArray in interface Collection<E>
      Overrides:
      toArray in class AbstractCollection<E>
      Type Parameters:
      T - the component type of the array to contain the collection
      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

    • iterator

      public Iterator<E> 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 weakly consistent.

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

    • spliterator

      public Spliterator<E> spliterator()
      Returns a Spliterator over the elements in this queue. The spliterator does not traverse elements in any particular order (the ORDERED characteristic is not reported).

      The returned spliterator is weakly consistent.

      The Spliterator reports Spliterator.SIZED and Spliterator.NONNULL.

      Specified by:
      spliterator in interface Collection<E>
      Specified by:
      spliterator in interface Iterable<E>
      Implementation Note:
      The Spliterator additionally reports Spliterator.SUBSIZED.
      Returns:
      a Spliterator over the elements in this queue
      Since:
      1.8

    • removeIf

      public boolean removeIf(Predicate<? super E> filter)
      Description copied from interface: Collection
      Removes all of the elements of this collection that satisfy the given predicate. Errors or runtime exceptions thrown during iteration or by the predicate are relayed to the caller.
      Specified by:
      removeIf in interface Collection<E>
      Parameters:
      filter - a predicate which returns true for elements to be removed
      Returns:
      true if any elements were removed
      Throws:
      NullPointerException - if the specified filter is null

    • removeAll

      public boolean removeAll(Collection<?> c)
      Description copied from class: AbstractCollection
      Removes all of this collection's elements that are also contained in the specified collection (optional operation). After this call returns, this collection will contain no elements in common with the specified collection.
      Specified by:
      removeAll in interface Collection<E>
      Overrides:
      removeAll in class AbstractCollection<E>
      Parameters:
      c - collection containing elements to be removed from this collection
      Returns:
      true if this collection changed as a result of the call
      Throws:
      NullPointerException - if this collection contains one or more null elements and the specified collection does not support null elements (optional) or if the specified collection is null
      See Also:

    • retainAll

      public boolean retainAll(Collection<?> c)
      Description copied from class: AbstractCollection
      Retains only the elements in this collection that are contained in the specified collection (optional operation). In other words, removes from this collection all of its elements that are not contained in the specified collection.
      Specified by:
      retainAll in interface Collection<E>
      Overrides:
      retainAll in class AbstractCollection<E>
      Parameters:
      c - collection containing elements to be retained in this collection
      Returns:
      true if this collection changed as a result of the call
      Throws:
      NullPointerException - if this collection contains one or more null elements and the specified collection does not permit null elements (optional) or if the specified collection is null
      See Also:

    • forEach

      public void forEach(Consumer<? super E> action)
      Description copied from interface: Iterable
      Performs the given action for each element of the Iterable until all elements have been processed or the action throws an exception. Actions are performed in the order of iteration, if that order is specified. Exceptions thrown by the action are relayed to the caller.

      The behavior of this method is unspecified if the action performs side-effects that modify the underlying source of elements, unless an overriding class has specified a concurrent modification policy.

      Specified by:
      forEach in interface Iterable<E>
      Parameters:
      action - The action to be performed for each element
      Throws:
      NullPointerException - if the specified action is null