The Open/Closed Principle states that software entities (classes, modules, etc.) should be open for extension, but closed for modification. What does this mean, and why is it an important principle of good object-oriented design?
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 extended without modifying its source code.
The Open-Close principle (OCP) is the O in the well known SOLID acronym. A module will be said to be open if it is still available for extension. For example, it should be possible to add fields to the data structures it contains, or new elements to the set of functions it performs.
The idea of open-closed principle is that existing, well-tested classes will need to be modified when something needs to be added. Yet, changing classes can lead to problems or bugs. Instead of changing the class, you simply want to extend it.
the open/closed principle is generally achieved by using inheritance and polymorphism.
It means that you should put new code in new classes/modules. Existing code should be modified only for bug fixing. New classes can reuse existing code via inheritance.
Open/closed principle is intended to mitigate risk when introducing new functionality. Since you don't modify existing code you can be assured that it wouldn't be broken. It reduces maintenance cost and increases product stability.
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