PriorityQueue (Java SE 22 & JDK 22)
Description: declaration: module: java.base, package: java.util, class: PriorityQueue
Keywords: java.util.PriorityQueue class

Module java.base
Package java.util

# Class PriorityQueue<E>

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

Type Parameters:
  `E` - the type of elements held in this queue

All Implemented Interfaces:
  `Serializable`, `Iterable<E>`, `Collection<E>`, `Queue<E>`

public class PriorityQueue<E> extends AbstractQueue<E> implements Serializable
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.

Since:
  1.5
See Also:
  - Serialized Form

- ## Constructor Summary

  Constructors
  Constructor
  Description
  `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(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(Comparator<? super E> comparator)`
  Creates a `PriorityQueue` with the default initial capacity and whose elements are ordered according to the specified comparator.
  `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.

- ## Method Summary

  Modifier and Type
  Method
  Description
  `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)`
  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.
  `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.
  `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 (optional operation).
  `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.
  `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

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

- ## Constructor Details

  - ### PriorityQueue

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

  - ### PriorityQueue

    public PriorityQueue(int initialCapacity)
    Creates a `PriorityQueue` 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

  - ### PriorityQueue

    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` - the comparator that will be used to order this priority queue. If `null`, the natural ordering of the elements will be used.
    Since:
      1.8

  - ### PriorityQueue

    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` - 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

  - ### PriorityQueue

    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` - 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

    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` - 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

    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` - 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

- ## Method Details

  - ### add

    public boolean add(E e)
    Inserts the specified element into this priority queue.
    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 this 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.
    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 this priority queue according to the priority queue's ordering
      `NullPointerException` - if the specified element is null

  - ### 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

  - ### 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>`
    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>`
    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 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 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()`.
    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.
    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

  - ### 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

  - ### clear

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

  - ### 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

  - ### comparator

    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:
      the comparator used to order this queue, or `null` if this queue is sorted according to the natural ordering of its elements

  - ### spliterator

    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.

    Specified by:
      `spliterator` in interface `Collection<E>`
    Specified by:
      `spliterator` in interface `Iterable<E>`
    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 (optional operation). 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:
      - `AbstractCollection.remove(Object)`
      - `AbstractCollection.contains(Object)`

  - ### 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:
      - `AbstractCollection.remove(Object)`
      - `AbstractCollection.contains(Object)`

  - ### 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