How can we describe T<S> as return type for a method using generics in java

Question

How can we describe T<S> as return type for a method

<T,S> T<S> getResult(Class<T> tClass, Class<S> sClass)

Answer 1

You can't, basically. There's no way of describing "a generic type with one type parameter" and using it like this.

Answer 2

Jon's right in that you can't do this in the general case. But if we think of a more specific case, say returning a specific type of List of a specific type of elements, we can do something like this:

<T, L extends List<T>> L getResult(Class<T> tClass, Class<L> lClass)

But then there's the problem of calling it. Classes are parameterized by raw types only. So if we wanted to call it like this:

ArrayList<String> result = getResult(String.class, ArrayList.class);

It wouldn't work, because ArrayList.class has type Class<ArrayList>, not Class<ArrayList<String>>.

But then we could use the super type token pattern, which Guava makes really easy with its TypeToken class:

<T, L extends List<T>> L getResult(Class<T> tClass, TypeToken<L> lType) {
   // use lType.getRawType()...
}

ArrayList<String> result = 
    getResult(String.class, new TypeToken<ArrayList<String>>(){});

Of course, this is only going to be of use if L represents a concrete class. It's also possible to create a TypeToken using type parameters, which won't help you at runtime.