Confused with Priority Queu's Behaviour

[Sagar Shroff]

As per my knowledge Priority Queue sort's on the bases of Natural Order.So if a Priority Queue is of String type then it will sort on the basis of a,b,c,d.....But just look at this code it doesn't agree....

public class Test
{
 public static void main(String args[])
 {
  PriorityQueue<String> pq=new PriorityQueue<String>();
  pq.add("c");
  pq.add("e");
  System.out.println(pq); [b]//c e
[/b]  System.out.println(pq.peek()+" peek "); //c
  System.out.println(pq); [b]//c e[/b]
  pq.offer("a");  
  System.out.println(pq); [b]//  a e c ...here it get's wrong instead of a c e it gives a e c...why soo ???[/b]
  pq.add("d"); 
  System.out.println(pq); [b]// a d c e[/b]
  pq.remove("d");  
  System.out.println(pq); [b]// a e c
[/b] }
}

Can someone please explain me why such behavior ...Or please correct me if ma concepts are wrong ......

[Sagar Shroff]

Please Help me Guys.............

[Stephan van Hulst]

PatienceIsAVirtue.

You're not asking pq to return elements. You're asking it to print itself. Look at the documentation for toString().

[Stoian Azarov]

Hello,
when you use System.out.prinln(pq) you are actually using pq.toString() method. PriorityQueue inherits this method from AbstractCollection class:

public String toString()
Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]").

And this is from PriorityQueue API:

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()).

[Sagar Shroff]

Stoian Azarov wrote:Hello,
when you use System.out.prinln(pq) you are actually using pq.toString() method. PriorityQueue inherits this method from AbstractCollection class:

public String toString()
Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]").

And this is from PriorityQueue API:

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()).

Thanks A Lot !!!.....Really Solved my mind's puzzle !!

[Sagar Shroff]

Stephan van Hulst wrote:PatienceIsAVirtue.

You're not asking pq to return elements. You're asking it to print itself. Look at the documentation for toString().

yeah i know i should have been a little patient....but was really anxious to know the answer....Thanks a lot Stephan !!

[dennis deems]

sagar shroff wrote:yeah i know i should have been a little patient....but was really anxious to know the answer.

A tip for those seeking instantaneous answers: Searching the forum may often yield results.

https://coderanch.com/t/267011/java-programmer-SCJP/certification/PriorityQueue
https://coderanch.com/t/270583/java-programmer-SCJP/certification/PriorityQueue
https://coderanch.com/forums/posts/list/473699#2121552