PriorityQueue In java

Some facts about PriorityQueue

  1) PriorityQueue keeps least value element at head, where ordering is determined using compareTo() method. It doesn't keep all elements in that order though, only head being least value element is guaranteed. This is in fact main difference between TreeSet and PriorityQueue in Java, former keeps all elements in a particular sorted order, while priority queue only keeps head in sorted order.

Example - [Watch: $200, Xbox: $300, IPone: $900, IPad: $1200]

  2) PriorityQueue  doesn't permit null elements and trying to add null elements will result in NullPointerException

  3) There is no "natural" ordering for objects which do not implement Comparable.. If you don't do any of these, the queue has no way to decide if the first element has a bigger priority than the second one.

In case of objects addition to priority queue, it must implements Comparable interface and must override compareTo() which specify condition which should fulfill ,where as in case of integer addition like below , it would give result comparing with natural ordering.

public class PriorityQueueInteger {

    

public static void main(String[] args) {

// Priority is given by default as natural ordering

PriorityQueue<Integer> queue= new PriorityQueue<Integer>();

     queue.add(50);

     queue.add(100);

     queue.add(20);

     queue.add(40);

     System.out.println(queue);

     }

}

Output  : [20, 40, 50, 100]