Open–closed principle

In object-oriented programming, the open/closed principle states "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification"; that is, such an entity can allow its behaviour to be modified without altering its source code. This is especially valuable in a production environment, where changes to source code may necessitate code reviews, unit tests, and other such procedures to qualify it for use in a product: code obeying the principle doesn't change when it is extended, and therefore needs no such effort.

The name Open/Closed Principle has been used in two ways. Both ways use inheritance to resolve the apparent dilemma, but the goals, techniques, and results are different.

Meyer's Open/Closed Principle
Bertrand Meyer is generally', which appeared in his 1988 book Object Oriented Software Construction. The idea was that once completed,

Polymorphic Open/Closed Principle
During the 1990s, the Open/Closed Principle became popularly redefined to refer to the use of abstracted interfaces, where the implementations can be changed and multiple implementations could be created and polymorphically substituted for each other.

In contrast to Meyer's usage, this definition advocates inheritance from abstract base classes. Interface specifications can be reused through inheritance but implementation need not be. The existing interface is closed to modifications and new implementations must, at a minimum, implement that interface.

Robert C. Martin's 1996 article "The Open-Closed Principle" was one of the seminal writings to take this approach. In 2001 Craig Larman related the Open/Closed Principle to the pattern by Alistair Cockburn called Protected Variations, and to the David Parnas discussion of information hiding.