How does Collections.reverseOrder know what type parameter to use while returning Comparator

Question

As per Java API spec, the signature of Collections.reverseOrder is

public static Comparator reverseOrder()

And the example gi

Answer 1

If you look at the signatures for Arrays.sort() you will see that the one you are calling has a signature of Arrays.sort(T[], Comparator), so you are in effect supplying a type parameter in the form of the array type you pass in.

EDIT: I made a small mistake here, and assumed that the call to reverseOrder() could infer the type, but without at least one parameter, no method could actually do that (see the link).

If the call to Collections.reverseOrder() took a parameter, then the method could infer the type from the call. There is an overloaded version of the method that takes a Comparator as an argument. We can demonstrate the inference in Eclipse with the following; hover over Collections.reverseOrder():

public class CollectionsFun {
   public static void main(String[] args) {
      String[] strs = new String[] {"foo", "bar", "baz"};
      Comparator comp = new Comparator() {
         @Override
         public int compare(String o1, String o2) {
            return o1.compareTo(o2);
         }        
      };
      Arrays.sort(strs, Collections.reverseOrder(comp));
      System.out.println(strs);
   }
}

Because the call to reverseOrder() in this example takes a typed parameter, the static method can infer the type that needs to be returned. However, when you call the overloaded form of the method that takes no parameter, no type can be inferred, so you get a Comparator