Can we use generic to allow only specific types instead of any type ?

Question

Suppose I have three isolated public classes (no IS-A relationship) A, B and C. I want to define a field in C such that it\'s type can either be A or B.

Currently I\'m a

Answer 1

Make the constructor private:

private C(T param){

And then provide static factory methods to create instances of particular types:

public static  C create(T param) {
  return new C<>(param);
}

public static  C create(T param) {
  return new C<>(param);
}

This doesn't prevent you from using the type C; you just can't create an instance of it.