Java generics, type erasure and type of a generic member

Java has type erasure and people say that one can't determine the type of a generic object at runtime without hacks. Consider the code below

    public class TestClass<T> {

    private T genericField;

    public TestClass(T genericField) {
        this.genericField = genericField;
    }

    public void printTypeInfo() {
        System.out.println("Hi I'm a " + genericField.getClass());
        System.out.println("Am I a string? " + (genericField instanceof String));
        System.out.println("Am I a long? " + (genericField instanceof Long));
    }

    public static void main(String [] args) {
        TestClass<String> genericString = new TestClass<>("Hello");
        TestClass<Long> genericLong = new TestClass<>(111111L);
        genericString.printTypeInfo();
        System.out.println("------------------");
        genericLong.printTypeInfo();
    }
}

It gives me the following result:

Hi I'm a class java.lang.String
Am I a string? true
Am I a long? false
------------------
Hi I'm a class java.lang.Long
Am I a string? false
Am I a long? true

Seems like type information is readily available at runtime. What am I missing here?