PriorityQueue

Queue is a collection of items in which only the earliest added item may be accessed. Basic operations are add (to the tail) or enqueue and delete (from the head) or dequeue. Delete returns the item removed. Also known as "first-in, first-out" or FIFO.
DeQueue is a queue to be any data structure that can have items inserted and removed from either end.

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
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() is 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()).

Following program is an example of priority queue

package collection.demos;



import java.util.PriorityQueue;



public class PriorityQueueDemo {



    public static void main(String[] args) {

        PriorityQueue<String> jobs = new PriorityQueue<String>();

        jobs.add("job-3");

        jobs.add("job-4");

        jobs.add("job-1");

        jobs.add("job-2");

        jobs.add("job-4");

        System.out.println("Queue : "+jobs);

        System.out.println("Element at head "+jobs.element());

        System.out.println("Removed element "+jobs.poll());

        System.out.println("Queue : "+jobs);

        jobs.add("job-1");

        System.out.println("Queue : "+jobs);

       

    }

}

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