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Encapsulate the Concept that Varies, i.e. a design is better when those parts that vary are encapsulated in a separate module.
This principle has two aspects that roughly correspond to the two sub-principles SRP and OCP. The first one is about making changes local. Everything which is supposed to change in the future should be encapsulated in a single module. This means cross-cutting concerns are avoided as much as possible. This is not completely possible but in many cases it is.
The second aspect is about introducing abstractions. Sometime the varying concept is one which varies at runtime rather than by maintenance. So at runtime it is decided upon a certain variation or there can be even several variations at the same time. In this case there has to be an abstract base class or an interface which encapsulates the varying concept. Several concrete descendant classes then specify the concrete variation.
The difference between the two aspects is whether the varying concept is one that changes over time during maintenance or one that may change at runtime. Nevertheless the advice is the same: encapsulate the concept that varies.
There are two reasons for this principle. The first reason is locality. When a varying concept is properly encapsulated in a single module, only this module is affected in case of a change. This reduces maintenance effort and ripple effects.
The second reason comes to play when the varying concept is implemented as an abstract class or interface. In this case a variation can be introduced without changing existing and tested code. This reduces testing effort (as already tested code does not need to be retested as it is not changed) as well as ripple effects (as the enhancement is done simply by adding a class. Note that for this rationale to work, the Liskov Substitution Principle also has to be adhered to.
Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides: Design Patterns: Elements of Reusable Object-Oriented Software (“GoF book”), p. 29