Home » openjdk-7 » java » util » [javadoc | source]
java.util
public class: PriorityQueue [javadoc | source]
java.lang.Object
   java.util.AbstractCollection<E>
      java.util.AbstractQueue<E>
         java.util.PriorityQueue

All Implemented Interfaces:
    java$io$Serializable, Queue, Collection

An unbounded priority {@linkplain Queue queue} based on a priority heap. The elements of the priority queue are ordered according to their {@linkplain Comparable natural ordering}, or by a Comparator provided at queue construction time, depending on which constructor is used. A priority queue does not permit {@code null} elements. A priority queue relying on natural ordering also does not permit insertion of non-comparable objects (doing so may result in {@code ClassCastException}).

The head of this queue is the least element with respect to the specified ordering. If multiple elements are tied for least value, the head is one of those elements -- ties are broken arbitrarily. The queue retrieval operations {@code poll}, {@code remove}, {@code peek}, and {@code element} access the element at the head of the queue.

A priority queue is unbounded, but has an internal capacity governing the size of an array used to store the elements on the queue. It is always at least as large as the queue size. As elements are added to a priority queue, its capacity grows automatically. The details of the growth policy are not specified.

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 priority queue in any particular order. If you need ordered traversal, consider using {@code Arrays.sort(pq.toArray())}.

Note that this implementation is not synchronized. Multiple threads should not access a {@code PriorityQueue} instance concurrently if any of the threads modifies the queue. Instead, use the thread-safe java.util.concurrent.PriorityBlockingQueue class.

Implementation note: this implementation provides O(log(n)) time for the enqueing and dequeing methods ({@code offer}, {@code poll}, {@code remove()} and {@code add}); linear time for the {@code remove(Object)} and {@code contains(Object)} methods; and constant time for the retrieval methods ({@code peek}, {@code element}, and {@code size}).

This class is a member of the Java Collections Framework.

Constructor:
 public PriorityQueue() 
 public PriorityQueue(int initialCapacity) 
 public PriorityQueue(Collection<? extends E> c) 
    Creates a {@code PriorityQueue} containing the elements in the specified collection. If the specified collection is an instance of a SortedSet or is another {@code 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
    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
 public PriorityQueue(PriorityQueue<? extends E> c) 
    Creates a {@code PriorityQueue} containing the elements in the specified priority queue. This priority queue will be ordered according to the same ordering as the given priority queue.
    Parameters:
    c - the priority queue whose elements are to be placed into this priority queue
    Throws:
    ClassCastException - if elements of {@code c} cannot be compared to one another according to {@code c}'s ordering
    NullPointerException - if the specified priority queue or any of its elements are null
 public PriorityQueue(SortedSet<? extends E> c) 
    Creates a {@code PriorityQueue} containing the elements in the specified sorted set. This priority queue will be ordered according to the same ordering as the given sorted set.
    Parameters:
    c - the sorted set whose elements are to be placed into this priority queue
    Throws:
    ClassCastException - if elements of the specified sorted set cannot be compared to one another according to the sorted set's ordering
    NullPointerException - if the specified sorted set or any of its elements are null
 public PriorityQueue(int initialCapacity,
    Comparator<? super E> comparator) 
    Creates a {@code PriorityQueue} 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.
    Throws:
    IllegalArgumentException - if {@code initialCapacity} is less than 1
Method from java.util.PriorityQueue Summary:
add,   clear,   comparator,   contains,   iterator,   offer,   peek,   poll,   remove,   removeEq,   size,   toArray,   toArray
Methods from java.util.AbstractQueue:
add,   addAll,   clear,   element,   remove
Methods from java.util.AbstractCollection:
add,   addAll,   clear,   contains,   containsAll,   isEmpty,   iterator,   remove,   removeAll,   retainAll,   size,   toArray,   toArray,   toString
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from java.util.PriorityQueue Detail:
 public boolean add(E e) 
    Inserts the specified element into this priority queue.
 public  void clear() 
    Removes all of the elements from this priority queue. The queue will be empty after this call returns.
 public Comparator<? super E> comparator() 
    Returns the comparator used to order the elements in this queue, or {@code null} if this queue is sorted according to the {@linkplain Comparable natural ordering} of its elements.
 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)}.
 public Iterator<E> iterator() 
    Returns an iterator over the elements in this queue. The iterator does not return the elements in any particular order.
 public boolean offer(E e) 
    Inserts the specified element into this priority queue.
 public E peek() 
 public E poll() 
 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).
 boolean removeEq(Object o) 
    Version of remove using reference equality, not equals. Needed by iterator.remove.
 public int size() 
 public Object[] toArray() 
    Returns an array containing all of the elements in this queue. The 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.

 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 the queue fits in the specified array with room to spare (i.e., the array has more elements than the queue), the element in the array immediately following the end of the collection is set to {@code 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().