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

You can't do that but you can set boundaries on what type you want to accept.

If you have

class A extends BaseType {}
class B extends BaseType {}

you can define the class C to be

class C<T extends BaseType> { ... }

Either a class or an interface as base type work.

Answer 2

Make the constructor private:

private C(T param){

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

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

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

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

Answer 3

You could use a marker interface for that:

interface AllowedInC {
    // intentionally empty because it will be used as a mere marker
}

class A implements AllowedInC {
    ...
}

class B implements AllowedInC {
    ...
}

class C<T extends AllowedInC> {
   ...
}

Only classes A or B (or another class implementing AllowedInC) will be useable in C<T>.