SOLID is a mnemonic acronym for five object-oriented designs
S - Single responsibility principle
O - Open-closed principle
L - Liskov substitution principle
I - Interface segregation principle
D - Dependency inversion principle
Single Responsibility
âA class should have one, and only one, reason to changeâ - Robert C. Martin
In other words a class should have only one responsibility - that ensures that separate responsibilities stay independent from each other
Open-closed Principle
âSoftware principles should be open for extension but closed for modificationâ
- Bertrand Meyer, âObject Oriented Software Constructionâ
You should be able to add new functionality to existing code without changing the existing code
Implementing the rule through inheritance leads to tight coupling, and if you donât want your implementations to share code itâs better implemented with interfaces, where interfaces are closed for implementation.
Liskov Substitution Principle
âLet Ί(x) be a property provable about objects x of type T. Then Ί(y) should be true for objects y of type S where S is a subtype of T.â
-Barbara Liskov
In simpler terms objects of superclass must be replaceable with objects of their subclasses without breaking the application
Interface Segregation Principle
âClients should not be forced to depend upon interfaces that they do not use.â
-Robert C. Martin
Code should not be forced to implement interfaces(and methods) it doesnât need. That is achieving by splitting interfaces into smaller, more specific ones so that clients only have to know about the methods that are of interest to them
It aims to reduce side effects of changes in codebase, by splitting software into multiple, independent parts.
Dependency Inversion
- High-level modules should not import anything from low-level modules. Both should depend on abstractions (e.g., interfaces).
- Abstractions should not depend on details. Details (concrete implementations) should depend on abstractions.
It is important that both high and low level modules depend on abstraction, splitting them with a layer of abstraction placed between them