Composition vs Inheritance

Question

I have a Validator interface with one method signature: public List<ParameterError> validateParameters(Parameters parameters); Various classes implement this interface.

There are also some common methods like isInteger , isInRange , etc. Should I have these methods in a ValidationHelper and use composition in the classes implementing the Validator interface, or should I make Validator an abstract class, put these methods there and use inheritance instead of composition?

Answer 1

I am assuming isInteger , isInRange etc. can be used even outside of the Validator classes if they are implemented generically. In that case, they should go into a separate util class, and that class should simply be called Utils rather than ValidationHelper . Classes/methods should always be named in the broadest context where you can use them.

Answer 2

Your helper methods are good candidates for static methods because they are pure functions and they are not expected to be polymorphic. You should use a utility class and put all such functions in it. This results in the least coupling: neither composition, nor inheritance; just pure dependency on the methods themselves.

Answer 3

I'd prefer composition over inheritance since the classes implementing Validator may not necessarily share an is-a relationship. They're more likely to be interested in the methods provided by the Validator interface.

Also, is it not possible to have the generic ( isInteger etc) methods implemented in the interface Validator and marked final ? They could even be made made static along the lines of this question.