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I have been struggling with the abstraction that lies between two classes when it comes to the issue of Inheritance or Composition...the IS-A vs HAS-A relationship between classes like LoginManager and AuthenticateManager.

The way I see it LoginManager can be either and can sit comfortably in a IS-A place with AuthenticateManager, inheriting it as a superclass (ie Class LoginManager extends AuthenticateManager) OR just declare AuthenticateManager's objects as members of its own class which would imply Composition, so i guess it just comes down to a thing of experience and the proper knowledge of the OOP paradigm. So please, can anyone help explain what a proper relationship would be between these classes?

PS: Please moderator don't close this topic as being inconsistent with the sites question asking principles.

Thanks.

Thilina H
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Cozzbie
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    I believe that `LoginManager` and `AuthenticationManager` are just related by the usage and not with ineheritence.The LoginManager should work on the basic user input and or any validations required on the data that we get from the user. Once everything is fine the data is passed to the AuthenticationManager to authenticate the user. The role of authmanager is to just validate the user and password with an existing datastore. – Nilesh Dec 25 '13 at 07:39
  • Wow...Nilish thanks...The way you have separated Validation from Authentication is insightful...and based on it I can see that a LoginManager Class doesn't have to inherit from a ValidationManager Class since not all validations have to do with logins – Cozzbie Dec 25 '13 at 08:00

2 Answers2

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It depends on your needs. Inheritance provides you with access to protected members of a superclass whereas composition does not. If there are class members that should be available only to a derived class then it does not make sense to make them properties and access them with composition because every other object could access them as well. Otherwise it is about what you and your team prefer.

EDIT:

No rules are so strict as you think they are. Design patterns, for instance, are just templates and you don't need to strictly follow them. You can't blindly follow rules, no one will credit you for this. It's much better to have OOP in mind but write your code so that you can clearly explain what have you done and why.

Since I don't know design of your two classes, I can't really tell you what are your needs. This statement (already mentioned) should be enough to point you in the right direction.

Inheritance provides you with access to protected members of a superclass whereas composition does not.

Ondrej Janacek
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  • Hmmm...now thats my point...OOP has rules that should be adhered to and your statement "it depends on my needs" kinda goes in contrast to the RULE of RULES...I should follow simple OOP tenets shouldn't I? Based on the RULES, what should my NEEDS be with respect to these type of classes? – Cozzbie Dec 25 '13 at 07:43
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Why not consider something that is easier to grasp?

I have a father - I inherit his genes (some good some bad?) But hey.

He had a kidney. His own and that made his composition.

I have my own kidney - my own and that makes my composition.

So

IS-A - I IS-A child of my father HAve-A - I hAVE-A kidney

My Kidney works differently to my fathers But I have blue eyes that I have inherited from my father

Ed Heal
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  • I know the difference between Inheritance and Composition. What I'm saying is that there isn't always a clean line between them or I'm not getting the clean clear cut line if to use Inheritance or Composition between an AuthenticateManager Class and LoginManager Class... – Cozzbie Dec 25 '13 at 07:45
  • @Cozzbie - Yes there is. Do you have to inherit stuff from the base class. Yes/No. If yes inheritance. If no that composition. – Ed Heal Dec 25 '13 at 07:48
  • @Cozzbie - it is called procreation – Ed Heal Dec 25 '13 at 08:58