What does SOLID aka (S.O.L.I.D) means?

S.O.L.I.D is an acronym for the first five object-oriented design(OOD) principles by Robert C. Martin, popularly known as Uncle Bob.

These principles, when combined together, make it easy for a programmer to develop software that are easy to maintain and extend. They also make it easy for developers to avoid code smells, easily refactor code, and are also a part of the agile or adaptive software development.

S.O.L.I.D stands for:

When expanded the acronyms might seem complicated, but they are pretty simple to grasp.

Let’s look at each principle individually to understand why S.O.L.I.D can help make us better developers.

Single-responsibility Principle

S.R.P for short – this principle states that:

A class should have one and only one reason to change, meaning that a class should have only one job.

Open-closed Principle

Objects or entities should be open for extension, but closed for modification.

Liskov substitution principle

Let q(x) be a property provable about objects of x of type T. Then q(y) should be provable for objects y of type S where Sis a subtype of T.

Interface segregation principle

A client should never be forced to implement an interface that it doesn’t use or clients shouldn’t be forced to depend on methods they do not use.

Dependency Inversion principle

The last, but definitely not the least states that:

Entities must depend on abstractions not on concretions. It states that the high level module must not depend on the low level module, but they should depend on abstractions.