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What is SOLID principles?


Introduction to SOLID principles
• S: Single Responsibility Principle (SRP)
• O: Open-closed Principle (OCP)
• L: Liskov substitution Principle (LSP)
• I: Interface Segregation Principle (ISP)
• D: Dependency Inversion Principle (DIP)


Definitions
• S: A class should only have a single responsibility, that is, only changes to one part of the software’s specification should be able to affect the specification of the class.
• O: Software entities (classes, modules, functions, etc.) should be open for extension but closed for modification
• L: Derived classes should be substitutable for their base classes without altering the correctness of the program.
• I: Clients should not be forced to depend on interfaces they do not use. Instead of one large interface, prefer many small, role-specific interfaces.
• D: High-level modules should not depend on low-level modules; both should depend on abstractions. Abstractions should not depend on details; details should depend on abstractions.


Principle Meaning Example
S – Single Responsibility Principle (SRP) A class should have only one reason to change. Each class should focus on a single responsibility. A UserService handles user logic, while EmailService handles email sending.
O – Open/Closed Principle (OCP) Software entities should be open for extension but closed for modification. Adding new payment methods by extending a base class PaymentProcessor instead of modifying it.
L – Liskov Substitution Principle (LSP) Subtypes must be substitutable for their base types without breaking functionality. A Square class should behave correctly when used in place of a Rectangle.
I – Interface Segregation Principle (ISP) Clients should not be forced to depend on interfaces they don’t use. Instead of one large interface, prefer many small ones. Instead of one large IMachine interface, split into smaller ones like IPrinter, IScanner.
D – Dependency Inversion Principle (DIP) Depend on abstractions, not on concrete implementations. High-level modules should not depend on low-level modules; both should depend on abstractions. A NotificationService depends on an INotifier interface, which can be implemented by EmailNotifier, SMSNotifier, etc.

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