Open-Closed Principle @ Robert 的部落格

OCP: the Open-Closed Principle
Software entities (class, modules, functions, etc.) should be open for extension, but closed for modification.

一個軟體個體應該要夠開放使得它可以被擴充，但是也要夠封閉以避免不必要的修改。這樣解釋鄉民們應該還是看不懂，接著看 Robert 在書中所說得這段話就會很清楚了 (p. 99)：

When a single change to a program results in a cascade of changes to dependent modules, the design smells of Rigidity. The OCP advises us to refactor the system so that further changes of that kind will not cause more modifications. If the OCP is applied well, then further changes of that kind are achieved by adding new code, not by changing old code that already works.