What Are the SOLID Principles?
SOLID is an acronym for five object-oriented design principles introduced by Robert C. Martin (Uncle Bob). These principles guide developers toward code that is easier to understand, extend, and maintain — reducing the risk of introducing bugs when requirements change.
The Five Principles at a Glance
- S — Single Responsibility Principle
- O — Open/Closed Principle
- L — Liskov Substitution Principle
- I — Interface Segregation Principle
- D — Dependency Inversion Principle
S — Single Responsibility Principle (SRP)
"A class should have only one reason to change."
Each class or module should do one thing well. If a class handles both user authentication and sending email notifications, it has two reasons to change. Splitting those responsibilities means a change to email logic can't accidentally break authentication.
In practice: If you find yourself writing "and" when describing what a class does, it probably violates SRP.
O — Open/Closed Principle (OCP)
"Software entities should be open for extension, but closed for modification."
You should be able to add new behavior without touching existing, tested code. This is typically achieved through abstraction — using interfaces or base classes that new implementations can extend.
In practice: Instead of adding if/else blocks every time a new payment method is supported, define a PaymentProcessor interface and create new classes for each method.
L — Liskov Substitution Principle (LSP)
"Objects of a subclass should be replaceable with objects of the superclass without breaking the application."
If you have a function that accepts a Bird and calls fly(), passing in a Penguin (which can't fly) violates LSP. Subtypes must honor the behavioral contract of their parent types.
In practice: Be careful with inheritance. If a subclass needs to override a method in a way that breaks expected behavior, reconsider the hierarchy.
I — Interface Segregation Principle (ISP)
"Clients should not be forced to depend on interfaces they do not use."
A fat interface with dozens of methods forces implementing classes to stub out methods they don't need. Split large interfaces into smaller, focused ones so that classes only implement what they actually use.
In practice: A Printer interface with print(), scan(), and fax() is problematic if your simple laser printer only prints. Split into Printable, Scannable, and Faxable.
D — Dependency Inversion Principle (DIP)
"Depend on abstractions, not concretions."
High-level modules (business logic) should not depend on low-level modules (database, file system). Both should depend on abstractions. This is the foundation of dependency injection frameworks.
In practice: Your OrderService should depend on an IOrderRepository interface, not on a specific MySQLOrderRepository class. Swap the database engine without touching business logic.
Do SOLID Principles Apply Beyond OOP?
While rooted in object-oriented design, the spirit of these principles applies broadly. Functional programmers apply SRP to function design, and DIP is echoed in module architecture across all paradigms. The core idea is always the same: reduce coupling, increase cohesion.
Getting Started
- Don't try to apply all five principles simultaneously when refactoring existing code — start with SRP.
- Write unit tests first; they naturally push you toward SOLID designs.
- Use code reviews to spot violations — a fresh pair of eyes quickly identifies "god classes" and tight coupling.
SOLID principles aren't rigid rules — they're guidelines that improve with practice. The more you apply them, the more naturally good design emerges in your code.