Question

I am developing a largish hierarchy of Java classes, some of which have a particular property that I am interested in querying at runtime (the property definitely only applies to classes, not specific instances).

I could create an abstract boolean method isFooBar() that subclasses could implement to indicate whether the property is present or not:

public abstract class MyBaseClass {
  ...
  public abstract boolean isFooBar();
}

Or alternatively I could use a marker interface FooBarProperty and do an instanceof check against the interface:

public class MyConcreteClass extends MyBaseClass implements FooBarProperty {
   ...
}

Or I guess you could even use an annotation as suggested by AmitD:

@FooBarAnnotation
public class MyConcreteClass extends MyBaseClass {
   ...
}

What are the pros and cons of each method, and which should normally be preferred?

Answer 1

Marker interfaces are inherited in class hierarchy, so they have limited capabilities (you cannot “remove” this marker interface for some particular subclass) and I cannot recommend them (Cloneable is a bad example of this). The question is – is your property really bound to object’s class, not to instance? If so, annotations are better, because they don’t add additional methods into your APIs and they are easier to maintain – you just add/remove the annotation (in case of checking method, you have to check all the subclasses, whether they override it or not).

EDIT: in your case, I would ask myself question: is my property like ‘is persistent’ (applies for all instances of given class) or ‘is visible’ (applies only for some instances)