TreeSet & Comparator

[Rekha Srinath]

Source: Inquisition

import java.util.*;
class test implements Comparator < test >
{
   int testNumber;
   public static void main( String args[] )
   {
      Set < test > s1 = new TreeSet < test > ();
      
      s1.add(new test());
      s1.add(new test());
 
   }
   public int compare( test t1 , test t2 )
   {
      return t1.testNumber - t2.testNumber;
   }
 
}

There is a ClassCastException on running this code which I thought is because of not properly setting the 'testNumber' instance variable, and compare() uses this unset variable for comparison.

So, I modified the code like this:

import java.util.*;
class test implements Comparator < test >
{
   test(int t) {
  this.testNumber = t;
   }
   int testNumber;
   public static void main( String args[] )
   {
      Set < test > s1 = new TreeSet < test > ();
 
      s1.add(new test(40));
      s1.add(new test(30));
 
   }
   public int compare( test t1 , test t2 )
   {
      return t1.testNumber - t2.testNumber;
   }
}

Even now, the same exception occurs. A TreeSet can have objects whose class implements Comparator interface, right? Then what is the issue here?

[Wouter Oet]

TreeSet(Comparator)

[Santiago Bravo]

Hi

yes a TreeSet can have objects whose class implements the Comparator interface, but you need to pass the class object to the Tree set. If you do something like this in main:

    
test q = new test(1);
Set < test > s1 = new TreeSet < test> (q);  

you won't get the exceptions as you are telling the TreeSet how you want to sort the set via the variable 'q'

If you had changed your code to implement Comparable and declare the compareTo() method then you won't need to pass anything to the TreeSet.


Hope that makes sense

[Harvinder Thakur]

A small change in the constructor of TreeSet()

new TreeSet(new test());

Your original code will run with the above change.
The rule being simple. If you want to sort using a comparator then you need to pass the comparator in the TreeSet's constructor. Otherwise by default TreeSet assumes that the objects in the Set implement the Comparable interface.

[Rekha Srinath]

Yes. I got it..
Thanks Wouter,Santiago and Harvinder !!!

[meera kanekal]

Rekha,
Could you please post the correct code. I was also trying to test the code you posted with the modifications suggested by the others.
Thanks,
Meera

[Chandra Bhatt]

meera,

See what harvinder says!

TreeSet constructor accepts Comparator (A class that implements Comparator).

new TreeSet<test>(new test());