SOLID Principles

SOLID is a set of five principles in object-oriented programming that help developers write maintainable, scalable, and flexible code. The principles were introduced by Robert C. Martin (Uncle Bob) and are commonly applied in software design, particularly in agile and clean code practices.

The SOLID Principles:

1. S - Single Responsibility Principle (SRP)

A class should have only one reason to change.

• Each class should only have one responsibility or function.
• This makes code easier to understand, maintain, and test.
• Example: Instead of a Report class handling both data storage and printing, split it into ReportData (handles data) and ReportPrinter (handles printing).

2. O - Open/Closed Principle (OCP)

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

• You should be able to extend functionality without modifying existing code.
• This is often achieved using inheritance and interfaces.
• Example: Instead of modifying a PaymentProcessor class to handle new payment methods, define an interface PaymentMethod and implement different payment classes (CreditCard, PayPal, etc.).

3. L - Liskov Substitution Principle (LSP)

Objects of a derived class must be substitutable for objects of the base class without altering correctness.

• A subclass should extend the behavior of a parent class, not break it.
• Example: If Bird has a fly() method, then a Penguin subclass should not override it in a way that causes errors. Instead, Bird should be redesigned to separate flying and non-flying birds.

4. I - Interface Segregation Principle (ISP)

Clients should not be forced to depend on interfaces they do not use.

• Large, general-purpose interfaces should be split into smaller, specific ones.
• Example: Instead of having one Worker interface with methods work() and eat(), separate it into Workable and Eatable, so that a Robot class doesn’t need to implement eat().

5. D - Dependency Inversion Principle (DIP)

High-level modules should not depend on low-level modules. Both should depend on abstractions.

• Instead of depending on concrete implementations, depend on abstractions (interfaces or abstract classes).
• Example: A DatabaseService class should depend on a DatabaseInterface, so it can switch from MySQL to PostgreSQL without changing the high-level logic.

Benefits

• Improves code maintainability and scalability.
• Reduces tight coupling, making it easier to modify or extend code.
• Enhances testability, as smaller, single-purpose classes are easier to unit test.