PriorityQueue Issue

[Chandra Bhatt]

PriorityQueue (todo list),


Case 1:

       System.out.println(pq); //pq is a PriorityQueue

Case 2:

       while(!(pq.isEmpty())) {
           System.out.println(pq.poll());
       }

Question:
Why does output from case 1 and case 2 mismatch?
Case 2, output is ordered on behalf of default (Comparable),
or custom(Comparator) todo way.

What way case 1 is following in printing?

Thanks,

cmbhatt
[ April 15, 2007: Message edited by: Chandra Bhatt ]

[Chandra Bhatt]

No Answer!

:roll:



cmbhatt

[Burkhard Hassel]

Hi ranchers,

API says about PriorityQueue:

The queue retrieval operations poll, remove, peek, and element access the element at the head of the queue.
(...)
The Iterator provided in method iterator() is not guaranteed to traverse the elements of the PriorityQueue in any particular order. If you need ordered traversal, consider using Arrays.sort(pq.toArray()).

The toString method of PQ uses the iterator, I guess.
The fact that PriorityQueue is ordered, but the iterator has no order frequently results in some confusion...
Can we call this a bug?

Example:

import java.util.*;
 
public class PriorityQueueCheck {
public static void main(String[] args) {
Queue<Task> pq = new PriorityQueue<Task>();
pq.offer(new Task(7));
pq.offer(new Task(5));
pq.offer(new Task(12));
pq.offer(new Task(-2));
System.out.println(pq);
System.out.print("iter: ");
Iterator<Task>it = pq.iterator();
while(it.hasNext())
System.out.print(it.next() + "\t");
System.out.print ("\npoll: ");
while(pq.isEmpty() == false)
System.out.print(pq.poll()  + "\t");
System.out.println();
}
}
 
class Task implements Comparable<Task>{
int priority;
Task(int priority){
this.priority=priority;
}
public int compareTo(Task that) {
return this.priority - that.priority;
}
public String toString(){
return "task(" + priority + ")";
}
}

prints:

task(-2), task(5), task(12), task(7)]
iter: task(-2)task(5)task(12)task(7)
poll: task(-2)task(5)task(7)task(12)

Yours,
Bu.

[Chandra Bhatt]

Thanks Burkhard,

I was lazy enough to give complete code! What you did greatly
here.

Your explanation is quite nice regarding the confusing issues.




Thanks,
cmbhatt

[Omer Haderi]

Hello there,

Burkhard Hassel is right the

  toString()  

method uses iterator. this is the code:

    
/**
     * 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
     * (<tt>"[]"</tt> :wink: .  Adjacent elements are separated by the characters
     * <tt>", "</tt> (comma and space).  Elements are converted to strings as
     * by <tt>String.valueOf(Object)</tt>.<p>
     *
     * This implementation creates an empty string buffer, appends a left
     * square bracket, and iterates over the collection appending the string
     * representation of each element in turn.  After appending each element
     * except the last, the string <tt>", "</tt> is appended.  Finally a right
     * bracket is appended.  A string is obtained from the string buffer, and
     * returned.
     * 
     * @return a string representation of this collection.
     */
    public String toString() {
StringBuffer buf = new StringBuffer();
buf.append("[");
 
        Iterator<E> i = iterator();
        boolean hasNext = i.hasNext();
        while (hasNext) {
            E o = i.next();
            buf.append(o == this ? "(this Collection)" : String.valueOf(o));
            hasNext = i.hasNext();
            if (hasNext)
                buf.append(", ");
        }
 
buf.append("]");
return buf.toString();
    }

you can download from SUN the src and check the API code for more info about API methods.

Cheers.