问题
I'm trying to refactor a class and set of subclasses where the M type does extend anything, even though we know it has to be a subclass of a certain type. That type is parametrized and I would like its parametrized types to be available to subclasses that already have values for M.
Is there any way to define this class without having to include the redundant K and V generic types in the parameter list. I'd like to be able to have the compiler infer them from whatever M is mapped to by subclasses.
public abstract class NewParametrized<K, V, M extends SomeParametrized<K, V>> {
public void someMethodThatTakesKAndV(K k1, V v1) { }
}
In other words, I'd like the class declaration to look something like:
public class NewParametrized<M extends SomeParametrized<K, V>> {
And K and V's types would be inferred from the definition of M.
回答1:
The problem is that K and V aren't really "repeated", at least not any more than a variable declaration and a use of that same variable are "repetition".
The way to think of this is that you declare the generic types K and V, and then you use those declared generic types in the definition of the generic type M. So, you don't declare the generic type K or V more than once: Rather, you declare each of them once and then refer to them in a declaration of something else.
To look at it another way, the number of generic types here is still three, namely K, V, and M. M happens to be defined in terms of K and V, but you didn't have to define M that way.
I should add that if you are a proponent of type inference, you might view the declaration and the use of the same variable as unnecessary repetition. However, if you're accustomed to programming in Java (or C, or C++, or many, many other languages), then you're probably accustomed to declaring variables before you use them.
回答2:
If it were possible, the declaration
public class NewParametrized<M extends SomeParametrized<K, V>> {
would be ambiguous if a K or V class were defined in the same package, it would be impossible to know whether it is a type variable or a reference to the concrete class.
来源：https://stackoverflow.com/questions/2382912/extract-generic-types-from-extended-generic