Question

I was surprised today when this code compiled:

class GenericClass<T> {
    public void emptyMethod(T instance) {
        // ..
    }

    public void print(T instance) {
        System.out.println(instance);
    }
}

public class Main {

    public static void main(String[] args) {
        GenericClass first = new GenericClass();
        System.out.println("Wow");
        first.emptyMethod(10);
        first.print(16);
    }

}

The compiler emits a warning (Type safety: The method emptyMethod(Object) belongs to the raw type GenericList. References to generic type GenericList should be parameterized), but anyway it does not cause a compiler error and it runs ‘fine’ (at least the provided print method). As I’m understanding, the compiler is using object as the type argument, but I find it counter-intuitive. Why would the compiler do such thing? Why it doesn’t require me to specify the type parameter?

Answer 1

You’re using a raw class, basically.

Think back to when generics were first introduced in Java: there was a load of code which already used List, ArrayList etc. In order to avoid breaking all of that code, but still reusing the existing classes, raw types were introduced – it’s basically using a generic type as if it weren’t one.

As you can see, you get a warning – so it’s worth avoiding – but that’s the primary reason for it being allowed at all.

See section 4.8 of the JLS for more information, which includes:

Raw types are closely related to wildcards. Both are based on existential types. Raw types can be thought of as wildcards whose type rules are deliberately unsound, to accommodate interaction with legacy code. Historically, raw types preceded wildcards; they were first introduced in GJ, and described in the paper Making the future safe for the past: Adding Genericity to the Java Programming Language by Gilad Bracha, Martin Odersky, David Stoutamire, and Philip Wadler, in Proceedings of the ACM Conference on Object-Oriented Programming, Systems, Languages and Applications (OOPSLA 98), October 1998.