how to implement a comparator for StringBuffer class in Java for use in TreeSet?

Question

I want to implement a Comparator for StringBuffer that compares the StringBuffer and adds to TreeSet accordingly. This is purely for learning purposes only. I know having a mutable object in a Hasable collection is a bad idea. but the purpose here is how to implement comparator for existing StringBuffer class of Java and use it to create a TreeSet. My current code is below. The code does not compile. kindly help me out here. Thanks

public class ComparatorExample {


    public 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>(new SbufferComparator());
                sb.add(one);
                sb.add(two);
                sb.add(three);
                System.out.println("set before change: "+ sb);
                one.append("onemore");
                System.out.println("set After change: "+ sb);
            }
        }

Answer 1

You should create the comparator class in a separate file not in the same class or make it static instead if you want to keep it inner.

    import java.util.Comparator;
    import java.util.Set;
    import java.util.TreeSet;

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>(new SbufferComparator());
            sb.add(one);
            sb.add(two);
            sb.add(three);
            System.out.println("set before change: "+ sb);
            one.append("onemore");
            System.out.println("set After change: "+ sb);
        }
    }

Pay attention to the import statements!