Introduction to Open / Closed Principle

Grokking SOLID Design Principles

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Introduction to Open / Closed Principle

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The Open/Closed Principle (OCP) is a core design principle in software development. It is defined as:

Software entities (such as classes, modules, and functions) should be open for extension, but closed for modification.

This means that your code should be designed in a way that allows new features to be added without changing the existing code. Let’s explore this further with a general example.

General Example: Gaming Console and Add-Ons

Imagine you own a gaming console. When you buy the console, it comes with a set of core functions, such as playing games, connecting to the internet, and using a standard controller. However, over time, you might want to enhance your gaming experience by adding new accessories like a steering wheel for racing games, a motion-sensing controller, or even a virtual reality (VR) headset.

The key idea here is that you do not need to change or modify the internal hardware or operating system of the console to use these accessories. The console is closed for modification, meaning its core functionality remains stable and intact. However, it is open for extension, as you can easily add new accessories to extend its capabilities and improve your experience.

In this way, the core system of the gaming console stays unchanged, while allowing for the addition of new features over time.

Explaining Open/Closed Principle (OCP)

Let’s now break down these two important terms:

Closed for Modification means that once a class, function, or module is written and tested, it should not need to be changed in the future to accommodate new behavior. By keeping the core code unchanged, you ensure that the tested and stable parts of the system remain reliable. If every change required modifying the existing code, you could introduce new bugs or break the functionality that has already been proven to work.
Open for Extension means that you can add new functionality to the system without modifying the existing code. This is typically achieved through techniques like inheritance, interfaces, or polymorphism, which allow you to extend the behavior of the code by adding new components (like subclasses or new methods) rather than modifying what’s already in place. This keeps the system flexible and adaptable while preserving the integrity of the original code.

By following this principle, your software remains stable, yet flexible enough to accommodate future changes or requirements without risking the functionality of the core components.

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