Getting generic parameter from supertype class

Question

Say I have a parent interface/class like so

interface Parent {}

And a number of implementing interfaces that fix the generic type.

Answer 1

I don't think so. Read about type erasure: the generic types are used only for compile-time checking, and then discarded. They're not stored in the compiled class files so they're not available at runtime.

Answer 2

Yes, despite what the others have said, this info is available if you have access to the subclass' Class object. You need to use getGenericSuperclass along with getActualTypeArguments.

ParameterizedType superClass = (ParameterizedType)childClass.getGenericSuperclass();
System.out.println(superClass.getActualTypeArguments()[0]);

In your example, the "actual" type argument should return the Class for Type.

Answer 3

If you need to do anything non-trivial with generic types at runtime, consider Guava's TypeToken. It can answer your question (and many more!) while addressing some of the nuanced concerns raised by commenters:

private interface Parent<T> {}
private interface Intermediate<U, V> extends Parent<V> {}
private interface Child<Z> extends Comparable<Double>, Intermediate<Z, Iterable<String>> {}

public void exploreGuavaTypeTokens() {
    final TypeToken<? super Child> token = TypeToken.of(Child.class).getSupertype(Parent.class);
    final TypeToken<?> resolved = token.resolveType(Parent.class.getTypeParameters()[0]);
    System.out.println(resolved); // "java.lang.Iterable<java.lang.String>"
    final Class<?> raw = resolved.getRawType();
    System.out.println(raw); // "interface java.lang.Iterable"
}