Overriding method that returns and consumes a generic class with sub-interfaces

Question

I want to make a generic interface that has two abstract methods, and one of them returns and consumes sub-type with same generic type of the interface.

The goal I want to achieve is that creating @FunctionalInterfaces having same parents, but different way of composing themselves.

My first approach is as below,

public interface ParentFunction<T, C extends ParentFunction> {

  void doSomething(T t);

  C<T> compose(C<T> other);
}

@FunctionalInterface
public interface SonFunction<T> extends ParentFunction<T, SonFunction> {

  @Override
  default SonFunction<T> compose(SonFunction<T> other){
    return null;
  }
}

@FunctionalInterface
public interface SonFunction<T> extends ParentFunction<T, SonFunction> {

  @Override
  default DaughterFunction<T> compose(SonFunction<T> other){
    return null;
  }
}

But an compile error occurs at C<T> of parent method saying, 'Type "C" does not have type parameters,' and another at @Override of child default method.

I can just separate my child interfaces without extending, but I hope them to have an super-type that the client code will only knows.

Is there any cool technique that I may use to achieve it?

Answer 1

In Java, you cannot do C<T>, however, you can require C to extend ParentFunction<T,C>

Same applies to your SonFunction and DaughterFunction.

Try this:

public interface ParentFunction<T, C extends ParentFunction<T, C>> {
    void doSomething(T t);
    C compose(C other);
}

@FunctionalInterface
public interface SonFunction<T> extends ParentFunction<T, SonFunction<T>> {

    @Override
    default SonFunction<T> compose(SonFunction<T> other){
        return null;
    }
}

@FunctionalInterface
public interface DaughterFunction<T> extends ParentFunction<T, DaughterFunction<T>> {

    @Override
    default DaughterFunction<T> compose(DaughterFunction<T> other){
        return null;
    }
}