• Object-Oriented Design
• API Design

Modules should be open for extension but closed for modification.

Beware that wrong application of OCP may lead to the anti-pattern onion.

See also section contrary principles.

Bertrand Meyer: Object-Oriented Software Construction, p. 57pp.

• Encapsulate the Concept that Varies (ECV): 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): The OCP demands introducing abstract base classes or interfaces.
• Keep It Simple Stupid (KISS): The OCP demands introducing abstract base classes or interfaces. This increases complexity.
• Model Principle (MP): OCP sometimes results in the introduction of artificial classes that do not correspond to a real-world concept.

• Dependency Inversion Principle (DIP): OCP results in the introduction of abstract classes or interfaces and descendant concrete classes. DIP now tells that other classes should only depend on the abstractions.
• Liskov Substitution Principle (LSP): OCP may results in the introduction of abstract classes or interfaces. Here it is important to get the abstraction right. Otherwise LSP may be violated.

Let's say the high-level module (your business logic), wants to be able to add or remove clients to the database. Instead of it talking to the database directly, it defines an interface called ClientRepository which contains the methods the business logic needs (DIP). Now you go along and implement a MySQLClientRepository. Some time in the future, you are asked to switch to an oracle database. You can now, without modifying any code from your business logic, switch to the oracle database: by extending ClientRepository to implement OracleClientRepository. You just need to wire an OracleClientRepository instance to the business logic and you have made the switch without modifying any business logic code.

Stub

• Robert C. Martin: Agile Software Development, Principles, Patterns, and Practices, p. 99pp.
• ButUncleBob: Principles of OOD

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