PriorityQueue()

[Rizvan Asgarov]

Dear friends,
I hope everything is OK...
Such question arose in me about PriorityQueue when i test myself with K&B book: (page-641, question:9)
When i debugged this code i have seen that's why changes 2 places with 4 when add 1 to pg?
So i have not understood this subject evidently well.. Can you explain it please..

import java.util.PriorityQueue;

public class PrQu {
    public static void main(String[] args) {

        PriorityQueue<String> pq = new PriorityQueue<String>();
        pq.add("2");
        pq.add("4");
        System.out.print(pq.peek() + " ");
        pq.offer("1");
        pq.add("3");
        pq.remove("1");
        System.out.print(pq.poll() + " ");
        if(pq.remove("2")) System.out.print(pq.poll() + " ");
        System.out.println(pq.poll() + " " + pq.peek());
    }
}

I thank at first
Rizvan
----------------------------------------------------------
My SCJP exam date approximately 15.03.2009

[Rizvan Asgarov]

I corrected mistakes in the top code
Also i changed this code as following:

    
          [code]
          pq.add("2");  
          pq.add("4"); 
          pq.add("1");  
          pq.add("3");
          pq.add("7");  
          pq.add("6"); 

..................
When debugging and I can explain so: adds 2, then 4, then 1 but I see 2 and 4 changed places (say myself-ok, maybe 4 is greater than 2) but at the end of debugging 6 has been located the most at the end.(it ought to 7) Why?
And can you explain it ( PriorityQueue rules without Comparator) please?

Thanks,
Rizvan
----------------------------------------------------------
My SCJP exam date approximately 15.03.2009

[Ankit Garg]

Rizvan I would recommend you to look at the PriorityQueue documentation once and then tell us if you have any doubts...

[Rizvan Asgarov]

Thanks a lot Ankit..
But i don't understand clearly.. therefore that write there ....depending on which constructor is used.(Comparable or Comparator)
But i have not used these there.. I used without them.
I don't understand that while adding numbers they sometimes chage places - sometimes doesn't change..
Can you help?

Thanks
Rizvan

[Ankit Garg]

Rizvan PriorityQueue always stores objects in sorted order. If you create a PriorityQueue with a Comparator, then it is used to sort the queue otherwise the default sorting is used to sort the objects in the queue. So if the PriorityQueue is not created with a Comparator and the objects added to the PriorityQueue don't implement Comparable, then you'll get a ClassCastException. Basically what I want to say is that whether created with a Comparator or not, PriorityQueue will always keep its elements sorted. Try out these simple programs
1.

import java.util.PriorityQueue;

public class PrQu {
  public static void main(String[] args) {

	PriorityQueue<String> pq = new PriorityQueue<String>();
	pq.add("2");  
	pq.add("4"); 
	pq.add("1");  
	pq.add("3");
	pq.add("7");  
	pq.add("6"); 
	while(pq.peek() != null) {
		System.out.println(pq.poll());
	}
  }
}

2.

import java.util.PriorityQueue;

public class PrQu {
  public static void main(String[] args) {

	PriorityQueue<String> pq = new PriorityQueue<String>();
	pq.add("2");  
	pq.add("4"); 
	pq.add("1");  
	pq.add("3");
	pq.add("7");  
	pq.add("6"); 
	for(String s: pq) {
		System.out.println(s);  //iterating over a priority queue results in unsorted elements
	}
  }
}

3.

import java.util.PriorityQueue;

public class PrQu {
  public static void main(String[] args) {

	PriorityQueue<PrQu> pq = new PriorityQueue<PrQu>();
	pq.add(new PrQu());  
	pq.add(new PrQu());  //ClassCastException
	for(PrQu s: pq) {
		System.out.println(s);
	}
  }
}

4.

import java.util.PriorityQueue;

public class PrQu implements Comparable<PrQu> {
  public static void main(String[] args) {

	PriorityQueue<PrQu> pq = new PriorityQueue<PrQu>();
	pq.add(new PrQu());  
	pq.add(new PrQu());  //no exception now
	for(PrQu s: pq) {
		System.out.println(s);
	}
  }
  public int compareTo(PrQu pr) {
	return 1;
  }
}

[Rizvan Asgarov]

Dear friend Ankit,

Thanks you very much..
Questions that I gave are easy although.. but forgive me that
a) I am new at Java for 6 months
b) I do not know English excellently
c) I hurry to get the certificate
Up to this day I have met with difficulties in two subject especially(regular expressions(have been explained well) and above mentioned)
So I believe to the Allah (God) and I rely on to myself...

I wish successes too, you..
Sincerely,
Rizvan

PS: You are further my real friend..
My SCJP exam date approximately 15.03.2009

[Ankit Garg]

Ohh so your exam is 3 days away. Best of luck dude . I hope Allah will listen to your prayers and you'll get a good score...