Hey, Thank you for your Explanation !

Correct me if i am wrong,

Suppose I have an ArrayList of some objects, to sort these objects I make use of Collection.sort(list),which in turn makes use of compare to () present inside the class that implements comparable interface.

Then what for Comparator inteferace is used ?

That is correct.

By implementing Comparator, your MyObject class gets a compareTo method which you can alter, otherwise it will just use the default compareTo method that's inside every `Object`

If you want to sort your objects according to your criteria, that logic needs to go into the compareTo method that you get from implementing Comparator. So if you're sorting according to your object's id, you can do something like this:

@Override
    public int compareTo(MyObject other){
        if (this.id == other.id) {
          return 0;
        } else if (this.id > other.id) {
          return 1;
        } else {
          return -1;
        }
    }