如何在类定义的通用类型中捕获子类型？

Question

I can't believe I cannot capture P without typing the class to a redundant 2-type class:

public class MyClass<T extends List<P>> {

   T getList(/**/){}
   P getRandomElement(){ /**/ }
}

Do I need, really, to define and instantiate MyClass as MyClass<String,ArrayList<String>> , couldn't it be inferred someway?

EDIT: What I mean is I see redundant having to define MyClass<P,T extends List<P>> because then I need to instantiate it always as MyClass<String,ArrayList<String>> , and carry String everywhere. It would be nice if the language allowed something like

MyClass<L extends List<P>> or similar. That way, MyClass<ArrayList<String>> would return an ArrayList<String> when executing getList() and an String when executing getRandomElement() , nicely.

Answer 1

If you really must have the exact type of List returned from getList , then you need the type parameter T as well as P .

public class MyClass<P, T extends List<P>> {

This way you can have a MyClass<String, ArrayList<String>> whose getList method returns an ArrayList<String> and a getRandomElement method that returns a String .

However, it is usually unnecessary to know the exact type of List . Usually, if you have a List , then it doesn't matter to the user of this class which kind of List is really is. In this case, you don't need the type parameter T , only P . You can change your class to this:

public class MyClass<P> {
   List<P> getList(/**/){}
   P getRandomElement(){ /**/ }
}

Then you can have a MyClass<String> whose getList method returns an List<String> and a getRandomElement method that returns a String .