Java generics: How to encode a Functor interface in Java?

Question

I want to define a Functor class in Java. This works:

//a Function
public interface F {
   public R apply(A a);
}

public interface Functor {         


        

Answer 1

Building on the answer of Sergey, I think I came close to what I wanted. Seems like I can combine his idea with my failed attempt:

public interface Functor> {
    public > I fmap(F f);
}

public class ListFunctor implements Functor> {
  final private List list;
  public ListFunctor(List list) {
     this.list = list;
  }

  @Override
  public >> I fmap(F f) {
     List result = new ArrayList();
     for(A a: list) result.add(f.apply(a));
     return (I) new ListFunctor(result);
  }
}

List list = java.util.Arrays.asList("one","two","three");
ListFunctor fs = new ListFunctor(list);
ListFunctor fi = fs.>fmap(stringLengthF);
//--> [3,3,5]

The remaining problem is that I could write e.g. ListFunctor fi = fs.> without complaints from the compiler. At least I can look for a way to hide the ugly guts behind a static method, and to enforce that relation behind the scenes...