S.O.L.I.D principles are five basic principles of object-oriented programming and design. They were first proposed by American software engineer Robert C. Martin in the early 2000s. The acronym S.O.L.I.D stands for Single responsibility, Open-closed, Liskov substitution, Interface segregation and Dependency inversion.

The main idea behind these principles is to make code more readable, understandable and maintainable. Let’s take a closer look at each principle.

History of the S.O.L.I.D Principles

The S.O.L.I.D principles were first proposed by American software engineer Robert C. Martin in the early 2000s in his paper “Design Principles and Design Patterns”. He came up with these principles as a way to make code more readable, understandable and maintainable. The S.O.L.I.D acronym was introduced later by Michael Feathers, a software developer and author. 

In the last 20 years, these five principles have revolutionized object-oriented programming. They changed how we write software and brought about a new era in which we can write code that is scalable and maintainable!

What are the S.O.L.I.D principles?

The S.O.L.I.D Principles serve the same purpose: To create understandable, readable, and testable code that many developers can collaboratively work on. The acronym S.O.L.I.D stands for:

  • Single responsibility principle
  • Open-closed principle
  • Liskov substitution principle
  • Interface segregation principle
  • Dependency inversion principle

The following principles can be used to develop software with considerations for maintaining and extending as the project grows. Adopting these practices will also contribute towards avoiding code smells, refactoring your codebase into a more maintainable state (refactorings are one way that this may happen), or even applying agile methodologies!

Let’s take a look at each of these principles in turn.

The Single Responsibility Principle

The single responsibility principle states that every class should have a single responsibility, and that responsibility should be entirely encapsulated by the class. In other words, do one thing and do it well.

The only change that should be able to affect the class’ specification is a software specification.

The Single Responsibility Principle states that every module should have only a single responsibility. This way, different teams can work on the same project but must be aware not to edit the class or else it will lead to incompatible modules.

Open-closed principle

The open-closed principle states that software entities (classes, modules, functions, etc.) should be open for extension but closed for modification. Modification is the act of changing or modifying something in an existing code, while extension means adding new features to your program.

That is, you should be able to extend a class’s behavior without having to modify the class itself. This means,we can add more functionality to the class extending it instead of modifying it

Modifying the existing code can introduce unexpected bugs. We should avoid this risk whenever possible by adding new functionality without touching already tested code that might be affected by these changes too closely for their own good!

Liskov substitution principle

The Liskov substitution principle states that subclasses should be substitutable for their superclasses. That is, a client should be able to use a subclass instead of its superclass without knowing the difference.

For example: class B is a subclass of A. That means that we should be able to replace an object from class A with an object from class B. The child extends the behavior.

Interface segregation principle

The interface segregation principle states that it is preferable to have many specific implementations that are few but too generic. In other words, an interface should be fine-grained and contain only those methods that are absolutely necessary for its clients.

Dependency inversion principle

The dependency inversion principle states that high-level modules should not depend on low-level modules. Both should depend on abstractions (e.g., interfaces). Abstractions should not depend on details. Details (concrete implementation) should depend on abstractions.

Final thoughts

There you have it! These are the S.O.L.I.D principles of object-oriented programming and design. If you follow these principles while designing, writing and refactoring your code then it will be much more clean. Your  testability also increases with this method since those are things that people usually focus on when they write tests for their program’s functionality in the first place!

We hope this article has helped to make these concepts more understandable and that you can start applying them in your own coding process. Thanks for reading!

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