Java - Alternatives to forcing subclass to have a static method

Question

I often find I want to do something like this:

class Foo{
public static abstract String getParam();
}

To force a subclasses of Foo to retur

Answer 1

It seems to me you want to solve the wrong problem with the wrong tool. If all subclasses define (can't really say inherit) your static method, you will still be unable to call it painlessly (To call the static method on a class not known at compile time would be via reflection or byte code manipulation).

And if the idea is to have a set of behaviors, why not just use instances that all implement the same interface? An instance with no specific state is cheap in terms of memory and construction time, and if there is no state you can always share one instance (flyweight pattern) for all callers.

If you just need to couple metadata with classes, you can build/use any metadata facility you like, the most basic (by hand) implementation is to use a Map where the class object is the key. If that suits your problem depends on your problem, which you don't really describe in detail.

EDIT: (Structural) Metadata would associate data with classes (thats only one flavor, but probably the more common one). Annotations can be used as very simple metadata facility (annotate the class with a parameter). There are countless other ways (and goals to achieve) to do it, on the complex side are frameworks that provide basically every bit of information designed into an UML model for access at runtime.

But what you describe (processors and parameters in database) is what I christened "set of behaviors". And the argument "parameters need to be loaded once per class" is moot, it completely ignores the idioms that can be used to solve this without needing anything 'static'. Namely, the flyweight pattern (for having only once instance) and lazy initialization (for doing work only once). Combine with factory as needed.