Why is this code throwing a ClassCastException and how to avoid it

Question

Consider this code:

import java.util.*;



class jm45 implements Comparator<jm45>
{
   private int x;
   jm45(int input) { x = input; }
   public static void main( String args[] )
   {
      List list = new ArrayList();
      list.add(new jm45(2));
      list.add(new jm45(2));
      Collections.sort(list); //faulty line
   }
   public int compare( jm45 t1 , jm45 t2 )
   {
      return t1.x - t2.x;
   }
}

Answer 1

Your class implements Comparator<jm45> instead of Comparable<jm45>.

A Comparator knows how to compare two objects - a Comparable knows how to compare another with itself.

You either need to pass in a comparator for sort() to use (as the second argument) or the values have to be comparable.

Here's a version which uses the Comparable interface instead:

import java.util.*;

class Test implements Comparable<Test>
{
    private int x;

    Test(int input)
    { 
        x = input;
    }

    public static void main(String args[])
    {
        List<Test> list = new ArrayList<Test>();
        list.add(new Test(2));
        list.add(new Test(2));
        Collections.sort(list);
    }

    public int compareTo(Test other)
    {
      return x - other.x;
    }
}

And here's a version which uses the Comparator interface:

import java.util.*;

class TestComparator implements Comparator<Test>
{
   public int compare(Test t1, Test t2)
   {
      return t1.getX() - t2.getX();
   }
}

class Test
{
    private int x;

    Test(int input)
    { 
        x = input;
    }

    int getX()
    {
        return x;
    }

    public static void main(String args[])
    {
        List<Test> list = new ArrayList<Test>();
        list.add(new Test(2));
        list.add(new Test(2));
        Collections.sort(list, new TestComparator());
    }
}

There's nothing to stop a class implementing Comparator<T> for itself, but it's a little strange for it to do so. For instance, you wouldn't normally ask one string to compare two other ones with each other - it's got nothing to do with the original string itself.