Overview of SOLID Principles
Get an introductory look at the five SOLID principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion.
Overview of SOLID Principles is a free Clean Architecture & Design Patterns in Practice lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Clean Architecture & Design Patterns in Practice learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
What are SOLID Principles?
The SOLID principles are five design guidelines for software that’s easy to maintain, understand, and extend. Think of them as a path to cleaner code.
Why SOLID Matters
Applying SOLID gives you code that’s easier to understand, simpler to test, more flexible to change, and friendlier to teamwork. It’s the bedrock of good OO design.
S: Single Responsibility Principle (SRP)
The Single Responsibility Principle: a class should have one reason to change. Don’t mix calculating data and saving it — split those concerns apart.
SRP in Action: Simple Report
This SimpleReport obeys SRP: its only job is generating report content. Printing and saving are someone else’s responsibility.
public class SimpleReport {
private String content;
public SimpleReport(String content) {
this.content = content;
}
// This class's single responsibility is to generate/represent the report content
public String generateReportContent() {
return "Report: " + content;
}
public static void main(String[] args) {
SimpleReport report = new SimpleReport("Sales Data for Q1");
System.out.println(report.generateReportContent());
}
}O: Open/Closed Principle (OCP)
The Open/Closed Principle: software should be open for extension but closed for modification. Add new behavior without editing working code.
OCP in Action: Flexible Greeters
Here a new greeter type slots in without touching the Greeter interface or existing classes — extending behavior, not modifying it. That’s OCP.
interface Greeter {
String greet();
}
class EnglishGreeter implements Greeter {
@Override
public String greet() {
return "Hello!";
}
}
class SpanishGreeter implements Greeter {
@Override
public String greet() {
return "¡Hola!";
}
}
public class OCPDemo {
public static void main(String[] args) {
Greeter english = new EnglishGreeter();
Greeter spanish = new SpanishGreeter();
System.out.println(english.greet());
System.out.println(spanish.greet());
}
}L: Liskov Substitution Principle (LSP)
The Liskov Substitution Principle: a subclass must be usable anywhere its parent is expected, without breaking the program. Subtypes honor the contract.
LSP in Action: Shapes
Any function taking a Shape handles both Rectangle and Circle correctly — they honor the contract. That’s LSP at work.
interface Shape {
double getArea();
}
class Rectangle implements Shape {
private double width;
private double height;
public Rectangle(double width, double height) {
this.width = width;
this.height = height;
}
@Override
public double getArea() {
return width * height;
}
}
class Circle implements Shape {
private double radius;
public Circle(double radius) {
this.radius = radius;
}
@Override
public double getArea() {
return Math.PI * radius * radius;
}
}
public class LSPDemo {
public static void printArea(Shape shape) {
System.out.println("Area: " + shape.getArea());
}
public static void main(String[] args) {
Shape myRectangle = new Rectangle(5, 4);
Shape myCircle = new Circle(3);
printArea(myRectangle);
printArea(myCircle);
}
}I: Interface Segregation Principle (ISP)
The Interface Segregation Principle: don’t force clients to depend on methods they don’t use. Prefer many small, focused interfaces over one fat one.
D: Dependency Inversion Principle (DIP)
The Dependency Inversion Principle: high-level and low-level modules should both depend on abstractions, not concrete details. That’s how you get loose coupling.
Check Your Understanding
Which of the following statements correctly describe the benefits of applying SOLID principles?
Recap: The Power of SOLID
You’ve met all five SOLID principles — single responsibility, open/closed, Liskov, interface segregation, dependency inversion — the toolkit for adaptable software.
Frequently asked questions
Is the “Overview of SOLID Principles” lesson free?
Yes — the full text of “Overview of SOLID Principles” is free to read here on the web, and the Clean Architecture & Design Patterns in Practice course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Clean Architecture & Design Patterns in Practice course, upgrade to CoddyKit PRO.
What will I learn in “Overview of SOLID Principles”?
Get an introductory look at the five SOLID principles: Single Responsibility, Open/Closed, Liskov Substitution, Interface Segregation, and Dependency Inversion. You practise Clean Architecture & Design Patterns in Practice with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Clean Architecture & Design Patterns in Practice?
No prior experience is required. Clean Architecture & Design Patterns in Practice on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Overview of SOLID Principles” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Clean Architecture & Design Patterns in Practice lesson?
Yes. Every Clean Architecture & Design Patterns in Practice lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Introduction to Clean Code
- Overview of SOLID Principles
- The Value of Good Design
- Cohesion, Coupling, and Separation of Concerns