How to design a common static method in all classes implementing an interface

Question

I have an interface called Relation, implemented by a class BasicRelation, and extended by subclasses (e.g. ParentChild, Sibling, Spouse). While developing my code, I realized that I often need a method which takes a String representation of a relation to create it. For example:

public class ParentChild implements Relation extends BasicRelation {

  // e.g. "Jack is Emily's father. Jill is her mother." will return the list
  // <ParentChild(Jack, Emily), ParentChild(Jill, Emily)>
  static List<ParentChild> fromSentence(String s) {
    ...
  }
}

Now, since I find myself needing this method (fromSentence(String)) in every class, except perhaps in BasicRelation, I would like to move it up the hierarchy. The problem is that the internal details of the method is subclass-dependent, so I can't have it as a static method in the interface Relation or the superclass BasicRelation.

Unfortunately, in Java, it is also not possible to have a static abstract method.

Is there any way to ensure that every subclass of BasicRelation (or every class implementing Relation) implements fromSentence(String)? If no, should I be designing this in a completely different way? I guess this last question is more of a request for design-advice than a question.

Answer 1

Why does the static method need to be in the interface? What's stopping you from having a 'Utility' class and having the method in there?

public class RelationUtility {
    public static BasicRelation relationFactory(String asString) {
        ....
    }
}

As a static method, there is no reason other than access to private members, which can also be accomplished by by 'default' permissions on those members....