Java.util.PriorityQueue Class

Introduction

The java.util.PriorityQueue class is an unbounded priority queue based on a priority heap.Following are the important points about PriorityQueue −

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.

Class declaration

Following is the declaration for java.util.PriorityQueue class −

pubpc class PriorityQueue<E>
   extends AbstractQueue<E>
   implements Seriapzable

Parameters

Following is the parameter for java.util.PriorityQueue class −

E − This is the type of elements held in this collection.

Class constructors

Sr.No.	Constructor & Description
1	PriorityQueue() This creates a PriorityQueue with the default initial capacity (11) that orders its elements according to their natural ordering.
2	PriorityQueue(Collection<? extends E> c) This creates a PriorityQueue containing the elements in the specified collection.
3	PriorityQueue(int initialCapacity) This creates a PriorityQueue with the specified initial capacity that orders its elements according to their natural ordering.
4	PriorityQueue(int initialCapacity, Comparator<? super E> comparator) This creates a PriorityQueue with the specified initial capacity that orders its elements according to the specified comparator.
5	PriorityQueue(PriorityQueue<? extends E> c) This creates a PriorityQueue containing the elements in the specified priority queue.
6	PriorityQueue(SortedSet<? extends E> c) This creates a PriorityQueue containing the elements in the specified sorted set.

Class methods

Sr.No.	Method & Description
1	boolean add(E e) This method inserts the specified element into this priority queue.
2	void clear() This method removes all of the elements from this priority queue.
3	Comparator<? super E> comparator() This method 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.
4	boolean contains(Object o) This method returns true if this queue contains the specified element.
5	Iterator<E> iterator() This method returns an iterator over the elements in this queue.
6	boolean offer(E e) This method inserts the specified element into this priority queue.
7	E peek() This method retrieves, but does not remove, the head of this queue, or returns null if this queue is empty.
8	E poll() This method retrieves and removes the head of this queue, or returns null if this queue is empty.
9	boolean remove(Object o) This method removes a single instance of the specified element from this queue, if it is present.
10	int size() This method returns the number of elements in this collection.
11	Object[] toArray() This method returns an array containing all of the elements in this queue.
12	<T> T[] toArray(T[] a) This method 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 inherited

This class inherits methods from the following classes −

java.util.AbstractQueue

java.util.AbstractCollection

java.util.Object

java.util.Collection