Question

Consider the following scenario:

class A<T> {}
class B extends A<Integer> {}

How can I know through reflection in Java that in B, the T type variable was instantiated as an Integer?

Answer 1

You can use

Type type = B.class.getGenericSuperclass();

// TODO: check with instanceof first?
ParameterizedType parameterized = (ParameterizedType) type;

// TODO: Check that there *are* type arguments
Type firstTypeArgument = parameterized.getActualTypeArguments()[0];

Short but complete example:

import java.lang.reflect.*;

class A<T> {}
class B extends A<Integer> {}

public class Test {
    public static void main(String[] args) {
        Type type = B.class.getGenericSuperclass();
        ParameterizedType parameterized = (ParameterizedType) type;
        // Prints class java.lang.Integer
        System.out.println(parameterized.getActualTypeArguments()[0]);
    }
}