Wednesday, 11 July 2018

Reasoning with SOLID Principles: Liskov Substitution

Liskov Substitution

Liskov is perhaps the most law like SOLID principle, every sub class should be useable as the base. I mean I can't find an example where I don't think this makes sense, probably more importantly now I wonder if I should be using inheritance at all?

The complex taxonomies of classes that made this relevant now seem distant in the past, and while inheritance still has much usefulness I feel that there was defiantly a time it was overused in the past. Better to compose objects of each other rather than to make them each other, looser coupling is implied in that relationship. i.e. a car has 4 wheels rather than is a four wheeled vehicle object.

This may seem a trivial difference but really comes into its own when single inheritance is enforced, composition is very take and choose what you want whereas inheritance implies a much deeper relationship, one cannot be a four wheeled vehicle and a two doored vehicle when it comes to base classes. whereas an object can have four wheels and two doors.

So Liskov good! But be careful with those large complex inheritence designs as inheritence is a extreemly tight form of coupling!

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