Question

I have an interface

public interface FooBar<T> { }

I have a class that implements it

public class BarFoo implements FooBar<Person> { }

With reflection, I want to take an instance of BarFoo and get that the version of FooBar it implements is Person.

I use .getInterfaces from BarFoo to get back to FooBar, but that doesn’t help me find out what T is.

Answer 1

You can grab generic interfaces of a class by Class#getGenericInterfaces() which you then in turn check if it’s a ParameterizedType and then grab the actual type arguments accordingly.

Type[] genericInterfaces = BarFoo.class.getGenericInterfaces();
for (Type genericInterface : genericInterfaces) {
    if (genericInterface instanceof ParameterizedType) {
        Type[] genericTypes = ((ParameterizedType) genericInterface).getActualTypeArguments();
        for (Type genericType : genericTypes) {
            System.out.println("Generic type: " + genericType);
        }
    }
}