The SOLID principles are five key guidelines in object-oriented programming and software design that make codebases more maintainable, scalable, and flexible. They serve as a foundation for writing clean code that adapts well to change.
The acronym stands for:
- 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)
Let's break each down.
Single Responsibility Principle (SRP)
A class should have one, and only one, reason to change.
If a class takes on multiple responsibilities, a change in one area might unintentionally affect the other. By limiting a class to one responsibility, you isolate change and keep the system easier to maintain.
Benefits
- Easier to test and debug.
- Code is more readable—each class has a clear purpose.
- Changes in one concern don't cascade into others.
Example
A User class handling both user data management and authentication violates SRP.
Instead, separate them into UserProfile and UserAuth classes. This way, changing authentication logic doesn't risk breaking profile-related logic.
Open-Closed Principle (OCP)
Software entities should be open for extension but closed for modification.
This principle emphasizes that you should be able to extend behavior without altering existing, tested code. Modifying core logic increases the chance of introducing bugs.
Benefits
- Stable, tested code remains untouched.
- New features are added without regressions.
- Promotes scalability and flexibility.
Example
Violates OCP (using if-else):
class Discount {
giveDiscount(customer: Customer): number {
if (customer.type === "Regular") {
return 10;
} else if (customer.type === "Premium") {
return 20;
} else if (customer.type === "Gold") {
return 30;
}
return 0;
}
}
Every new customer type requires modifying this class.
Follows OCP (using polymorphism):
interface Customer {
giveDiscount(): number;
}
class RegularCustomer implements Customer {
giveDiscount(): number {
return 10;
}
}
class PremiumCustomer implements Customer {
giveDiscount(): number {
return 20;
}
}
class Discount {
giveDiscount(customer: Customer): number {
return customer.giveDiscount();
}
}
Now, adding a GoldCustomer requires creating a new class—no modifications to Discount.
👉 For a deeper dive, see this example.
Liskov Substitution Principle (LSP)
Subtypes must be substitutable for their base types.
If class S is a subtype of class T, then objects of type T should be replaceable with objects of type S without breaking the program.
Why It Matters
- Prevents violations of expected behavior.
- Keeps inheritance hierarchies safe and predictable.
Example resources:
Interface Segregation Principle (ISP)
Clients should not be forced to depend on methods they do not use.
Large, “fat” interfaces make implementing classes unnecessarily complex. Instead, split big interfaces into smaller, role-specific ones.
Benefits
- Reduces coupling.
- Easier to refactor.
- Classes only implement what they need.
These smaller interfaces are often called role interfaces because they capture a specific role rather than everything at once.
Dependency Inversion Principle (DIP)
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.
What It Means
- Business logic (high-level modules) should not directly depend on infrastructure or utilities (low-level modules).
- Both should communicate through interfaces or abstractions.
- This makes swapping implementations (e.g., changing from MySQL to PostgreSQL) much easier.
👉 Detailed breakdown: Cloudaffle guide
Wrapping Up
The SOLID principles may seem abstract at first, but they boil down to reducing coupling, increasing cohesion, and making change safe.
- SRP: One responsibility per class.
- OCP: Extend without modifying.
- LSP: Subtypes should be safe replacements.
- ISP: Small, specific interfaces.
- DIP: Depend on abstractions, not implementations.
By following these principles, you not only write cleaner, more modular code, but also prepare your system to handle growth and inevitable change with minimal pain.