Java generics - method parameter

Question

Is it necessary to parametrize the entire interface for this scenario, even though Bar is only being used in a single method?

public interface IFoo<T>{

    void method1(Bar<T> bar);

    //Many other methods that don't use Bar....

}  

public class Foo1 implements IFoo<Yellow>{

    void method1(Bar<Yellow> bar){...};

    //Many other methods that don't use Bar....

}


public class Foo2 implements IFoo<Green>{

    void method1(Bar<Green> bar){...};

    //Many other methods that don't use Bar....

}

Answer 1

You're not extending the interface. Is that deliberate? You can do this:

public class Foo2 implements IFoo<Green> {
  void method1(Bar<Green> bar);
}

Just doing this:

public class Foo<Green> {
  void method1(Bar<Green> bar);
}

won't compile.

Answer 2

I would ask the question a bit differently, because the need suggests a cost, which is not actual. I don't think it actually matter if it is used on only one, or several methods.

When you make several calls to the instance, how does the type parameter vary?:

• if constant once you instantiated the instance, you parameterize the entire interface.
• if it may be different on each call, you parameterize the method.

That way, the type of parameter actually gives information about the code, improve the meaning and clarity.

---

Edited: Example

If sometimes, the type parameter varies from call to call, for the same instance ...
It has to be a method parameter.

Answer 3

No, it's not necessary from a syntactic standpoint. You can also do this:

public interface IFoo {

  <T> void method1(Bar<T> bar);

  /* Many other methods that don't use Bar…  */

}

Or this:

public interface IFoo {

  void method1(Bar<?> bar);

  /* Many other methods that don't use Bar…  */

}

The correct choice depends on the semantics of IFoo and what its implementations are likely to do with the Bar instances they receive through method1.