passing comparator to collections.sort() in Java?

Question

I have the following code where I have a Treeset, to which if I pass my comparator it works fine. however, if I construct my Treeset and then call collections.sort, I get compile error. my code is here

import java.util.*;

public class ComparatorExample {
private static class SbufferComparator implements Comparator<StringBuffer> {

        @Override
        public int compare(StringBuffer s1, StringBuffer s2) {
            return s1.toString().compareTo(s2.toString());

        }

}


    public static void main(String[] args) {
            StringBuffer one = new StringBuffer("one");
            StringBuffer  two = new StringBuffer("two");
            StringBuffer three = new StringBuffer("three");
            Set<StringBuffer> sb=new TreeSet<StringBuffer>();
             //The below line works
            //Set<StringBuffer> sb=new TreeSet<StringBuffer>(new SbufferComparator());
            sb.add(one);
            sb.add(two);
            sb.add(three);
            System.out.println("set before change: "+ sb);
            //This does not work
            Collections.sort(sb, new SbufferComparator());
            System.out.println("set After change: "+ sb);
        }
    }

PS. I know StringBuffer is a bad type to keep as element in Set. However, I was testing if Java allows to keep a mutable object in Set. (python does not allow mutable object to placed in set or dictionary(map))

Answer 1

Collections.sort() can only be applied to a List, and you are passing a Set so it fails (it should not compile at all).

TreeSet is a sorted Set, so you should create it with an appropriate Comparator and the content of the set will always be sorted, without the need to manually sort it.

Answer 2

Collections.sort expects a List rather than a Set. Try this instead

Set<StringBuffer> sb=new TreeSet<StringBuffer>(new SbufferComparator());

and remove the call to sort completely