Adding to a Generic Set passed into a method

Question

Let us say I have the following classes:

• Animal
• Cat
• Dog
• Cow

Animal is the base class, cat, dog, and cow each subclass it.

I now have a Set<Cat>, Set<Dog> and Set<Cow> each of these are used in the same way, so it makes sense to make a generic function to operate on them:

private boolean addObject(Animal toAdd, Animal defVal, Set<? extends Animal> vals)

This works great, I can freely pass in my Sets with no problem.

The problem appears: I can't attempt to add the Animal toAdd to parameter vals. Googling reveals that if I change the method to read:

private boolean addObject(Animal toAdd, Animal defVal, Set<? super Animal> vals)

I will be able to add the Animal to parameter vals. This works, except now, I can't pass in my subclassed my Sets of Cats, Dogs, and Cows. Further research tells me that the following works, with no warnings to boot:

private <T> boolean addObject(T toAdd, T defVal, Set<? super T> vals)

The problem being, I need to be able to perform method calls that Animal's all have. This is easily gotten around using a simple cast:

((Animal)toAdd).getAnimalType()

Is there any way around this problem so I can keep the generic functionality, and not require casting? Aside from making my Sets all Sets of the Base Type, Animal in the case of this example?

Answer 1

Good research, you were almost there. You just need to add an upper bound to T.

private <T extends Animal> boolean addObject(T toAdd, T defVal, Set<? super T> vals)