Question

I’ve got a set of Java classes I’m using as a toolkit that I can plug into many projects. I do not want to change the code in this toolkit, so that any time I update it or fix a bug, I can just drop it into any of my projects that are using it, without needing to worry about local changes. Thus, if any local projects need to override a method in the toolkit, I just make a local version of the toolkit object, like such:

FILE: toolkit/Dog()

public class Dog(){
  public void pet(){
    print("scratch ruff");
  }
}

FILE: local/Dog()

public class Dog extends toolkit/Dog {
  public void pet(){
    print("rub ears");
  }
}

And in the local objects, I refer to the local Dog object instead of the toolkit Dog object.

This so far has worked great, but I just ran into an issue. Another class in the toolkit uses Dog.

FILE: toolkit/DogHandler

public void careForPack( List<Dog> arg_allTheDogs ){
  for( Dog fido : arg_allTheDogs ){
    fido.pet();
  }
}

The problem that arises is that the system doesn’t like that these are not the same Dog object. I don’t want to locally override DogHandler, because I’ll just end up overriding my entire toolkit, which rather defeats the purpose.

Is there a way for the DogHandler to recognize Dog‘s child, also called Dog, as being valid?

Answer 1

You should make your toolkit method accept a List containing objects of a type that extends toolkit.Dog. You can use ? extends T for this:

public void careForPack( List<? extends toolkit.Dog> arg_allTheDogs ){
  for( toolkit.Dog fido : arg_allTheDogs ){
    fido.pet();
  }
}