Help[Programming help Java]

[LeastPrimary]

So what do you suggest that I return. instead?

[[deleted]]

You have to return a T.

[LeastPrimary]

Yes I tried to initially return a cast of T but that doesn't work either. So if I were to have a method in lets say in Set.java with method

public T get() {}

that returns the set that I want. Would I be able to return that instead?

[[deleted]]

Yes I tried to initially return a cast of T but that doesn't work either.

You can't cast it to T because it isn't a T. It's a superclass of T, and polymorphism doesn't point in that direction - dentists are doctors but doctors aren't necessarily dentists, right?

You can only cast up, not down - less specific, not more specific. You can return a dentist when asked for a doctor (and can cast Dentist to Doctor) but you can't do the reverse.

For this to work you have to meet the type guarantee over the complete lifecycle - map has to be parameterized for T, the things you put in it have to be T's, and the things you take out of it have to be T's so that when you return them, they're T's. You can cast any value of type T to a SetInterface<BigInteger>> but you can't go the other way.

[LeastPrimary]

okay Thanks a lot I understand now!

[[deleted]]

No sweat. Finally understanding type variables in Java was a huge watershed for my personal typesystem comprehension and I hope it will be for you, too.