Added in API level 1

Summary: Ctors | Methods | Inherited Methods

PriorityQueue

public class PriorityQueue
extends AbstractQueue<E> implements Serializable

java.lang.Object
↳	java.util.AbstractCollection<E>
	↳	java.util.AbstractQueue<E>
		↳	java.util.PriorityQueue<E>

An unbounded priority queue based on a priority heap. The elements of the priority queue are ordered according to their natural ordering, or by a Comparator provided at queue construction time, depending on which constructor is used. A priority queue does not permit null elements. A priority queue relying on natural ordering also does not permit insertion of non-comparable objects (doing so may result in 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 poll, remove, peek, and 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() and the Spliterator provided in method spliterator() are not guaranteed to traverse the elements of the priority queue in any particular order. If you need ordered traversal, consider using Arrays.sort(pq.toArray()).

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

Implementation note: this implementation provides O(log(n)) time for the enqueuing and dequeuing methods (offer, poll, remove() and add); linear time for the remove(Object) and contains(Object) methods; and constant time for the retrieval methods (peek, element, and size).

This class is a member of the Java Collections Framework.

Summary

Public constructors
`PriorityQueue()` Creates a `PriorityQueue` with the default initial capacity (11) that orders its elements according to their natural ordering.
`PriorityQueue(int initialCapacity)` Creates a `PriorityQueue` with the specified initial capacity that orders its elements according to their natural ordering.
`PriorityQueue(Comparator<? super E> comparator)` Creates a `PriorityQueue` with the default initial capacity and whose elements are ordered according to the specified comparator.
`PriorityQueue(int initialCapacity, Comparator<? super E> comparator)` Creates a `PriorityQueue` with the specified initial capacity that orders its elements according to the specified comparator.
`PriorityQueue(Collection<? extends E> c)` Creates a `PriorityQueue` containing the elements in the specified collection.
`PriorityQueue(PriorityQueue<? extends E> c)` Creates a `PriorityQueue` containing the elements in the specified priority queue.
`PriorityQueue(SortedSet<? extends E> c)` Creates a `PriorityQueue` containing the elements in the specified sorted set.

Public methods
`boolean`	`add(E e)` Inserts the specified element into this priority queue.
`void`	`clear()` Removes all of the elements from this priority queue.
`Comparator<? super E>`	`comparator()` Returns the comparator used to order the elements in this queue, or `null` if this queue is sorted according to the natural ordering of its elements.
`boolean`	`contains(Object o)` Returns `true` if this queue contains the specified element.
`void`	`forEach(Consumer<? super E> action)`
`Iterator<E>`	`iterator()` Returns an iterator over the elements in this queue.
`boolean`	`offer(E e)` Inserts the specified element into this priority queue.
`E`	`peek()`
`E`	`poll()`
`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)`
`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.
`final Spliterator<E>`	`spliterator()` Creates a late-binding and fail-fast `Spliterator` over 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.
`Object[]`	`toArray()` Returns an array containing all of the elements in this queue.

Inherited methods

From class


        
          java.util.AbstractQueue

`boolean`	`add(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
`boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this queue.
`void`	`clear()` Removes all of the elements from this queue.
`E`	`element()` Retrieves, but does not remove, the head of this queue.
`E`	`remove()` Retrieves and removes the head of this queue.

From class


        
          java.util.AbstractCollection

`boolean`	`add(E e)` Ensures that this collection contains the specified element (optional operation).
`boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`void`	`clear()` Removes all of the elements from this collection (optional operation).
`boolean`	`contains(Object o)` Returns `true` if this collection contains the specified element.
`boolean`	`containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>`	`iterator()` Returns an iterator over the elements contained in this collection.
`boolean`	`remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`boolean`	`retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`abstract int`	`size()` Returns the number of elements in this collection.
`<T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`Object[]`	`toArray()` Returns an array containing all of the elements in this collection.
`String`	`toString()` Returns a string representation of this collection.

From class


        
          java.lang.Object

`Object`	`clone()` Creates and returns a copy of this object.
`boolean`	`equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void`	`finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>`	`getClass()` Returns the runtime class of this `Object`.
`int`	`hashCode()` Returns a hash code value for the object.
`final void`	`notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void`	`notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String`	`toString()` Returns a string representation of the object.
`final void`	`wait(long timeoutMillis, int nanos)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait(long timeoutMillis)` Causes the current thread to wait until it is awakened, typically by being notified or interrupted, or until a certain amount of real time has elapsed.
`final void`	`wait()` Causes the current thread to wait until it is awakened, typically by being notified or interrupted.

From interface


        
          java.util.Queue

`abstract boolean`	`add(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions, returning `true` upon success and throwing an `IllegalStateException` if no space is currently available.
`abstract E`	`element()` Retrieves, but does not remove, the head of this queue.
`abstract boolean`	`offer(E e)` Inserts the specified element into this queue if it is possible to do so immediately without violating capacity restrictions.
`abstract E`	`peek()` Retrieves, but does not remove, the head of this queue, or returns `null` if this queue is empty.
`abstract E`	`poll()` Retrieves and removes the head of this queue, or returns `null` if this queue is empty.
`abstract E`	`remove()` Retrieves and removes the head of this queue.

From interface


        
          java.util.Collection

`abstract boolean`	`add(E e)` Ensures that this collection contains the specified element (optional operation).
`abstract boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`abstract void`	`clear()` Removes all of the elements from this collection (optional operation).
`abstract boolean`	`contains(Object o)` Returns `true` if this collection contains the specified element.
`abstract boolean`	`containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`abstract boolean`	`equals(Object o)` Compares the specified object with this collection for equality.
`abstract int`	`hashCode()` Returns the hash code value for this collection.
`abstract boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`abstract Iterator<E>`	`iterator()` Returns an iterator over the elements in this collection.
`default Stream<E>`	`parallelStream()` Returns a possibly parallel `Stream` with this collection as its source.
`abstract boolean`	`remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`abstract boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`default boolean`	`removeIf(Predicate<? super E> filter)` Removes all of the elements of this collection that satisfy the given predicate.
`abstract boolean`	`retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`abstract int`	`size()` Returns the number of elements in this collection.
`default Spliterator<E>`	`spliterator()` Creates a `Spliterator` over the elements in this collection.
`default Stream<E>`	`stream()` Returns a sequential `Stream` with this collection as its source.
`abstract <T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.
`abstract Object[]`	`toArray()` Returns an array containing all of the elements in this collection.
`default <T> T[]`	`toArray(IntFunction<T[]> generator)` Returns an array containing all of the elements in this collection, using the provided `generator` function to allocate the returned array.

From interface


        
          java.lang.Iterable

`default void`	`forEach(Consumer<? super T> action)` Performs the given action for each element of the `Iterable` until all elements have been processed or the action throws an exception.
`abstract Iterator<E>`	`iterator()` Returns an iterator over elements of type `T`.
`default Spliterator<E>`	`spliterator()` Creates a `Spliterator` over the elements described by this `Iterable`.

Public constructors

PriorityQueue

Added in API level 1

public PriorityQueue ()

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

PriorityQueue

Added in API level 1

public PriorityQueue (int initialCapacity)

Creates a PriorityQueue with the specified initial capacity that orders its elements according to their natural ordering.

Parameters
`initialCapacity`	`int`: the initial capacity for this priority queue

Throws
`IllegalArgumentException`	if `initialCapacity` is less than 1

PriorityQueue

Added in API level 24

public PriorityQueue (Comparator<? super E> comparator)

Creates a PriorityQueue with the default initial capacity and whose elements are ordered according to the specified comparator.

Parameters
`comparator`	`Comparator`: the comparator that will be used to order this priority queue. If `null`, the natural ordering of the elements will be used.

PriorityQueue

Added in API level 1

public PriorityQueue (int initialCapacity, 
                Comparator<? super E> comparator)

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

Parameters
`initialCapacity`	`int`: the initial capacity for this priority queue
`comparator`	`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

PriorityQueue

Added in API level 1

public PriorityQueue (Collection<? extends E> c)

Creates a PriorityQueue containing the elements in the specified collection. If the specified collection is an instance of a SortedSet or is another 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`	`Collection`: 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

PriorityQueue

Added in API level 1

public PriorityQueue (PriorityQueue<? extends E> c)

Creates a 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`	`PriorityQueue`: the priority queue whose elements are to be placed into this priority queue

Throws
`ClassCastException`	if elements of `c` cannot be compared to one another according to `c`'s ordering
`NullPointerException`	if the specified priority queue or any of its elements are null

PriorityQueue

Added in API level 1

public PriorityQueue (SortedSet<? extends E> c)

Creates a 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`	`SortedSet`: 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 methods

add

Added in API level 1

public boolean add (E e)

Inserts the specified element into this priority queue.

Parameters
`e`	`E`: the element to add

Returns
`boolean`	`true` (as specified by `Collection#add`)

Throws
`ClassCastException`	if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's ordering
`NullPointerException`	if the specified element is null

clear

Added in API level 1

public void clear ()

Removes all of the elements from this priority queue. The queue will be empty after this call returns.

comparator

Added in API level 1

public Comparator<? super E> comparator ()

Returns the comparator used to order the elements in this queue, or null if this queue is sorted according to the natural ordering of its elements.

Returns
`Comparator<? super E>`	the comparator used to order this queue, or `null` if this queue is sorted according to the natural ordering of its elements

contains

Added in API level 1

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).

Parameters
`o`	`Object`: object to be checked for containment in this queue

Returns
`boolean`	`true` if this queue contains the specified element

forEach

Added in API level 24

public void forEach (Consumer<? super E> action)

Parameters
`action`	`Consumer`

Throws
`NullPointerException`

iterator

Added in API level 1

public Iterator<E> iterator ()

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

Returns
`Iterator<E>`	an iterator over the elements in this queue

offer

Added in API level 1

public boolean offer (E e)

Inserts the specified element into this priority queue.

Parameters
`e`	`E`

Returns
`boolean`	`true` (as specified by `Queue#offer`)

Throws
`ClassCastException`	if the specified element cannot be compared with elements currently in this priority queue according to the priority queue's ordering
`NullPointerException`	if the specified element is null

peek

Added in API level 1

public E peek ()

Returns
`E`

poll

Added in API level 1

public E poll ()

Returns
`E`

remove

Added in API level 1

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).

Parameters
`o`	`Object`: element to be removed from this queue, if present

Returns
`boolean`	`true` if this queue changed as a result of the call

removeAll

Added in API level 1

public boolean removeAll (Collection<?> c)

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.

Parameters
`c`	`Collection`: collection containing elements to be removed from this collection

Returns
`boolean`	`true` if this collection changed as a result of the call

Throws
`NullPointerException`

removeIf

Added in API level 24

public boolean removeIf (Predicate<? super E> filter)

Parameters
`filter`	`Predicate`

Returns
`boolean`

Throws
`NullPointerException`

retainAll

Added in API level 1

public boolean retainAll (Collection<?> c)

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.

Parameters
`c`	`Collection`: collection containing elements to be retained in this collection

Returns
`boolean`	`true` if this collection changed as a result of the call

Throws
`NullPointerException`

size

Added in API level 1

public int size ()

Returns the number of elements in this collection. If this collection contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

Returns
`int`	the number of elements in this collection

spliterator

Added in API level 24

public final Spliterator<E> spliterator ()

Creates a late-binding and fail-fast Spliterator over the elements in this queue. The spliterator does not traverse elements in any particular order (the ORDERED characteristic is not reported).

The Spliterator reports Spliterator#SIZED, Spliterator#SUBSIZED, and Spliterator#NONNULL. Overriding implementations should document the reporting of additional characteristic values.

Returns
`Spliterator<E>`	a `Spliterator` over the elements in this queue

toArray

Added in API level 1

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 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().

Parameters
`a`	`T`: 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
`T[]`	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

toArray

Added in API level 1

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.

Returns
`Object[]`	an array containing all of the elements in this queue