Java annotation for enforcing static variables or static methods?

Question

I'm interested in doing something like this:

public interface Foo {

  public static "abstract" Bar SOME_BAR; // subclasses define one of these

}

and

public interface Foo {

  public static "abstract" Baz buildABas(...); // subclasses define this method 

}

without the statics, this is OOP 101, but it can't be done in standard oop java. I wonder if there's an annotation that would ensure this behavior?

edit:

i'm interested in specifying a set of options which define how to set things for "configurable" objects. this could be command-line flags, etc.

Answer 1

I'm guessing what you're wanting is to have a method like

public void callFoo(Class<?> clazz)

and you want to ensure that clazz has a method public static void foo() .

I thought about this a while and none of the techniques that come to mind will get you there. You can use an AnnotationProcessor to ensure that any classes annotated with a certain annotation have a specific method or what have you (and generate a compile error if they don't) but there's no way to ensure (at compile time) that Class arguments passed to callFoo(Class<?> clazz) are annotated with your annotation.

Here's an AnnotationProcessor that gets you halfway there:

import java.util.Set;

import javax.annotation.processing.AbstractProcessor;
import javax.annotation.processing.Messager;
import javax.annotation.processing.RoundEnvironment;
import javax.annotation.processing.SupportedAnnotationTypes;
import javax.annotation.processing.SupportedSourceVersion;
import javax.lang.model.SourceVersion;
import javax.lang.model.element.Element;
import javax.lang.model.element.ElementKind;
import javax.lang.model.element.TypeElement;
import javax.tools.Diagnostic;


@SupportedSourceVersion(SourceVersion.RELEASE_6)
@SupportedAnnotationTypes("so.Foo")
public class FooAnnotationProcessor extends AbstractProcessor {

    @Override
    public boolean process(Set<? extends TypeElement> annotations,
            RoundEnvironment roundEnv) {

        TypeElement foo = processingEnv.getElementUtils().getTypeElement("so.Foo");
        Set<? extends Element> classes = roundEnv.getElementsAnnotatedWith(foo);
        Messager messenger = processingEnv.getMessager();
        for (Element e : classes) {
            boolean found = false;
            for (Element method : e.getEnclosedElements()) {
                messenger.printMessage(Diagnostic.Kind.ERROR, 
                        method.getSimpleName());
                if (method.getKind() == ElementKind.METHOD && method.getSimpleName().toString().equals("getInstance")) {
                    found = true;
                    break;
                }
            }
            if (!found)
            {
                messenger.printMessage(Diagnostic.Kind.ERROR, 
                    "The following class does not implement getInstance : " + e.getSimpleName(),e);
            }
        }
        return true;
    }

}

Ultimately, I would suggest you either allow it to be something enforced at runtime or your redesign your code so you don't need to use static methods.