• Object-Oriented Design
• API Design
• Architecture

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.

• Introduce a separate module for the concept that may change in the future. In that way the future change will only affect that particular module. If the varying concept is properly encapsulated, only this module will have to change.
• Introduce an interface encapsulating the varying concept. The interface may be implemented differently by several classes and code that only relies on the interface can handle any class implementing the interface. In case of another variation, just another class has to be introduced and this class has to implement the interface. If the abstraction is done properly, no module has to change.
• Introduce an abstract base class encapsulating the varying concept. This is basically the same as introducing an interface. But here, implementation can also be inherited. So common parts can remain in the abstract base class whereas only the actual variations are defined in the subclasses. By means of method overriding, the implementation of the base class methods can be changed without touching the base class directly.
• Use design patterns. Several design patterns use the above techniques to encapsulate varying concepts. For example:
  • Abstract Factory: A family of objects changes.
  • Factory Method: The exact type of an object to create changes.
  • Adapter: The interface of a module changes.
  • Bridge: A concept varies in more than one aspect.
  • Decorator: The behavior of certain methods may need to be enhanced.
  • Iterator: The traversal algorithm of a structure changes. Or the structure itself changes resulting in the need for a different traversal algorithm.
  • Observer: The objects interested in a certain event may change.
  • State: The behavior in a certain state or the state machine (states and transitions) of a certain module changes.
  • Strategy: An algorithm changes.
  • Template Method: The concrete steps in an algorithms change.
  • Visitor: New operations have to be added to a given more or less static inheritance structure of classes.
  • …

See section contrary principles.

The principle is stated, explained and used in the GoF book:

Erich Gamma, Richard Helm, Ralph Johnson, John Vlissides: Design Patterns: Elements of Reusable Object-Oriented Software (“GoF book”), p. 29

But the idea if ECV is actually much older. It was first presented in

David Parnas: On the Criteria To Be Used in Decomposing Systems into Modules

• Accepted: This principle was popularly described in the GoF book and can thus be regarded accepted.
• Examined: Many of the patterns in the GoF book are precisely about encapsulating varying concepts. See strategies section.

• Single Responsibility Principle (SRP): A responsibility in the sense of SRP is defined as “a reason for change”. This is a concept that varies over time. SRP tells that each module should have exactly one responsibility, i.e. the concept/responsibility should be encapsulated in that module.
• Open-Closed Principle (OCP): The OCP demands encapsulating abstract concepts in base classes (or interfaces) in order to be able to enhance the module by subclassing which is possible without changing the previously written code. In this case several variations of a concept may exist in the code at the same time. There is always the abstract base class plus one or usually more concrete subclasses. So the OCP is about encapsulating abstract concepts that vary “in space”.

• More Is More Complex (MIMC): ECV demands adding a new class for a new varying concept.
• Model Principle (MP): ECV sometimes results in classes which do not correspond to a real-world concept in the sense of MP. A “concept that varies” can also be a technical concept.

• Low Coupling (LC): ECV results in the creation of a new module. When introducing such a new module, LC has to be adhered to.
• Liskov Substitution Principle (LSP): ECV may result in the introduction of an abstract base class. Here it is important to get the abstraction right. Otherwise LSP may be violated.
• Generalization Principle (GP): Encapsulating a varying concept typically results in a more generally applicable solution. This is especially true when an abstract concept is encapsulated by introducing an interface or an abstract class.
• Dependency Inversion Principle (DIP): ECV may result in the introduction of an abstract base class. Here DIP demands that other classes should only depend on this new abstract base class and not on the concrete subclasses.
• Information Hiding/Encapsulation (IH/E): ECV tells that varying concepts should be encapsulated. IH/E then tells how encapsulation is done.

Incomplete

• Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides: Design Patterns: Elements of Reusable Object-Oriented Software
• Single Responsibility Principle
• Open-Closed Principle

Discuss this wiki article and the principle on the corresponding talk page.