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Design Points - Subclassing vs Subtyping

This article explores the differences between subclassing and subtyping in design, specifically focusing on the implementation of a Stack. It discusses the challenges and consequences of using subclassing and proposes a better approach using subtyping. The article also touches on the concepts of inheritance, polymorphism, and code reuse in relation to subclassing and subtyping.

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Design Points - Subclassing vs Subtyping

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  1. Design Points - Subclassing vs Subtyping Stéphane Ducasse --- 2010

  2. A little problem • How to implement a Stack?

  3. Stack subclass of OrderedCollection • Stack>>pop • ^ self removeFirst • Stack>>push: anObject • self addFirst: anObject • Stack>>top • ^ self first • Stack>>size, Stack>>includes: • are free, inherited from

  4. Wait! but • What do we do with all the rest of the interface of OrderedCollection? • a Stack IS NOT an OrderedCollection! • We cannot substitute an OrderedCollection by a Stack • Some messages do not make sense on Stack • Stack new addLast: anObject • Stack new last • So we have to block a lot of methods...

  5. Consequences... • Stack>>removeFirst • self shouldNotImplement • Stack>>pop • ^ super removeFirst

  6. The Problem • There is not a simple relationship between Stack and OrderedCollection • Stack interface is not an extension or subset of OrderedCollection interface • Compare with CountingStack a subclass of Stack • CountingStack interface is an extension of Stack interface

  7. Compare the two uses

  8. Compare the two replacements

  9. A Better Approach • By defining the class Stack that uses OrderedCollection • Object subclass: Stack • instVarNames: ‘elements’ • Stack>>push: anElement • elements addFirst: anElement • Stack>>pop • element isEmpty ifFalse: [^ element removeFirst]

  10. Inheritance and Polymorphism • Polymorphism works best with conforming/substituable interfaces • Inheritance creates families of classes with similar interfaces • An abstract class describes an interface fulfilled by its subclasses • Inheritance helps software reuse by making polymorphism easier

  11. About subtyping and subclassing • You have only extends or subclass: in programming language. • But they can be used to define a subclassing or subtyping relationship.

  12. Subtyping Inheritance • Reuse of specifications • A subclass refines superclass specifications • A program that works with Numbers will work with Fractions. • A program that works with Collections will work with Arrays.

  13. Subclassing • Inheritance for code reuse • Dictionary is a subclass of Set • Semaphore is a subclass of LinkedList • No relationship between the interfaces of the classes • Subclass reuses code from superclass, but has a different specification. It cannot be used everywhere its superclass is used. Usually overrides a lot of code. • ShouldNotImplement use is a bad smell…

  14. Inheritance for Code Reuse • Inheritance for code reuse is good for • rapid prototyping • getting application done quickly. • Bad for: • easy to understand systems • reusable software • application with long life-time.

  15. Subtyping Essence • You reuse specification • You should be able to substitute an instance by one of its subclasses (more or less) • There is a relationship between the interfaces of the class and its superclass

  16. How to Choose? • Favor subtyping • When you are in a hurry, do what seems easiest. • Clean up later, make sure classes use “is-a-subtype” relationship, not just “is-implemented-like”. • Is-a-subtype is a design decision, the compiler only enforces is-implemented-like!!!

  17. Quizz • Circle subclass of Point? • Poem subclass of OrderedCollection?

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