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PriorityBlockingQueue


java.util.concurrent

Class PriorityBlockingQueue<E>

  • Type Parameters:
    E - the type of elements held in this collection
    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() 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>> {
       static final AtomicLong seq = new AtomicLong(0);
       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;
       }
     }
    Since:
    1.5
    See Also:
    Serialized Form
    • Constructor Summary

      Constructors
      Constructor and Description
      PriorityBlockingQueue()
      Creates a PriorityBlockingQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
      PriorityBlockingQueue(Collection<? extends 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, Comparator<? super E> comparator)
      Creates a PriorityBlockingQueue with the specified initial capacity that orders its elements according to the specified comparator.
    • Method Summary

      Methods
      Modifier and Type Method and 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.
      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.
      int size()
      Returns a count of how many objects this Collection contains.
      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.
      String toString()
      Returns the string representation of this Collection.
    • Constructor Detail

      • 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 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
    • Method Detail

      • 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
      • 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 class: AbstractCollection
        Returns a count of how many objects this Collection contains.

        In this class this method is declared abstract and has to be implemented by concrete Collection implementations.

        Specified by:
        size in interface Collection<E>
        Specified by:
        size in class AbstractCollection<E>
        Returns:
        how many objects this Collection contains, or Integer.MAX_VALUE if there are more than Integer.MAX_VALUE 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 Collection<E>
        Specified by:
        remove in interface BlockingQueue<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 Collection<E>
        Specified by:
        contains in interface BlockingQueue<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
      • 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
      • toString

        public String toString()
        Description copied from class: AbstractCollection
        Returns the string representation of this Collection. The presentation has a specific format. It is enclosed by square brackets ("[]"). Elements are separated by ', ' (comma and space).
        Overrides:
        toString in class AbstractCollection<E>
        Returns:
        the string representation of this Collection.
      • 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
      • 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>
        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 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 Iterable<E>
        Specified by:
        iterator in interface Collection<E>
        Specified by:
        iterator in class AbstractCollection<E>
        Returns:
        an iterator over the elements in this queue


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