Java abstract static Workaround

Question

I understand that neither a abstract class nor an interface can contain a method that is both abstract and static because of ambiguity problems, but is there a workaround?

Answer 1

A type system allows you to express some constraints among types, but it's limited. That's why javadocs are littered with constraints in human language, asking people to follow rules that the compiler cannot check.

if you want to extend it beyond what language provides natively, you can write your own static analysis tool. that is not uncommon. for example: findbug. also IDEs do that too, they checking thing beyond what language dictates. you can write a plug in to enforce that a subclass must have a static method of such signature.

in your case, it's not worth it. have javadoc in the superclass urge implementors to include a static method, that's good enough.

I'll provide a convoluted way of expressing your constraint anyway, but DO NO DO IT. people get really carried away of make everything checkable at compile time, at the price of making code unreadable.

interface WidgetEnumerator
{
    List getAllWidgets();
}

public class Abs
{
    static List getAllWidgets(Class clazz){ ... }
}

public class Sub extends Abs
{
}

public class SubWidgetEnumerator implements WidgetEnumerator
{
    public List getAllWidgets() { ... }
}

How it works: for any subclass of Abs, it is forced to provide an implementation of WidgetEnumerator. subclass author cannot forget that. Now invocation Abs.getAllWidgets(Sub.class) contains sufficient information to resolve that implementation, i.e. SubWidgetEnumerator. It is done through reflection, but it is type safe, there are no string literals involved.