Making a generic comparator class

1. You can't write a single comparator for everything without some assumptions on what the types will be. What do you do with custom classes? How can you decide which one is greater than the other? For more classes in the wild, a comparator does not make sense.

2. On the other hand, if you restrict yourself to String, Integer, Double, then they are Comparable and you can simply write the comparator with the compareTo() method:

   public int compare(T element1,T element2)
   {
       return element1.compareTo(element2);
   }
   

but then you would simply use the natural order of elements, and it would defeat the purpose of using a comparator. You usually don't need one in these cases.

The closest thing you can do to this is a Comparator that can compare any objects that implement the Comparable interface:

class NaturalComparator<T extends Comparable<T>> implements Comparator<T> {
  public int compare(T a, T b) {
    return a.compareTo(b);
  }
}

That's really the closest you can do: only Comparable objects have the "natural ordering" you're trying to model here. But generally, once you have Comparable objects, you don't necessarily need a Comparator: for example, Collections.sort can take either a List with a Comparator, or a List with Comparable elements.