priorityQuery problem

[varinder mahajan]

import java.util.*;
class Demo
{
public static void main( String args[] )
{
PriorityQueue<Integer> p=new PriorityQueue<Integer>();
p.offer(1);
p.offer(1);
p.offer(1);
System.out.println("size:"+p.size());

for(int i )
{
System.out.println("poll");
p.poll();
System.out.println("size:"+p.size());
}
System.out.println("added two elements");
p.offer(1);
p.offer(1);
System.out.println("size:"+p.size());
for(int i )
{
System.out.println("peek");
p.peek();
System.out.println("size:"+p.size());
}
}
}

C:\>java Demo
size:3
poll
size:2
Exception in thread "main" java.util.ConcurrentModificationException

This is the output what is the problem
at java.util.PriorityQueue$Itr.next(Unknown Source)
at Demo.main(Demo.java:12)

[Vijitha Kumara]

You cannot modify the Collection with the enhanced for loop. Here you are removing elements (poll()) from the Queue within the enhanced for loop. Try using Iterator instead.
[ September 04, 2008: Message edited by: Vijitha Kumara ]

[Ankit Garg]

Oops...His code has Smileys!!!

[Jesper de Jong]

Please use code tags (instead of quoting) when you post Java code.

[varinder mahajan]

import java.util.*;
class Demo
{
public static void main( String args[] )
{
PriorityQueue<Integer> p=new PriorityQueue<Integer>();
p.offer(1);
p.offer(1);
p.offer(1);
System.out.println("size:"+p.size());

for(int i : p)
{
System.out.println("poll");
p.poll();
System.out.println("size:"+p.size());
}
System.out.println("added two elements");
p.offer(1);
p.offer(1);
System.out.println("size:"+p.size());
for(int i : p)
{
System.out.println("peek");
p.peek();
System.out.println("size:"+p.size());
}
}
}

[varinder mahajan]

Please help, i am unable to understand . where is the problem

[Ankit Garg]

I read somewhere that if you add or delete any element from a queue, then any iterator created before the insertion or deletion is invalidated and calling any method on the iterator will cause ConcurrentModificationException

[Vijitha Kumara]

import java.util.*;
class Demo
{
public static void main( String args[] ) 
{
PriorityQueue<Integer> p=new PriorityQueue<Integer>();
p.offer(1);
p.offer(1);
p.offer(1);
System.out.println("size:"+p.size());
[B]Iterator ip = p.iterator();
 
while(ip.hasNext()){[/B]
System.out.println("poll"); 
p.poll();
System.out.println("size:"+p.size());
}
 
System.out.println("added two elements");
p.offer(1);
p.offer(1);
System.out.println("size:"+p.size());
for(int i :-P	   ) //Ok to use enhanced for loop (you are not 
//modifying anything)
{ 
System.out.println("peek"); 
p.peek();
System.out.println("size:"+p.size());
}
}
} 

Hope this helps