Types of principles in software engineering ?

Overview

Software engineering principles are guidelines that help write maintainable, scalable, and clean code. The most important ones are SOLID, DRY, KISS, YAGNI, and others.

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1. SOLID Principles

S — Single Responsibility Principle

A class should have only one reason to change.

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// ❌ Bad — UserService does too much
class UserService {
  Future<User> fetchUser(int id) async { ... }
  void sendEmail(String email) { ... }       // Wrong place
  void saveToDatabase(User user) { ... }     // Wrong place
}

// ✅ Good — split responsibilities
class UserService { Future<User> fetchUser(int id) async { ... } }
class EmailService { void sendEmail(String email) { ... } }
class UserRepository { void save(User user) { ... } }

O — Open/Closed Principle

Open for extension, closed for modification.

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abstract class Shape { double area(); }
class Circle extends Shape { @override double area() => 3.14 * r * r; }
class Rectangle extends Shape { @override double area() => w * h; }
// Add new shapes without modifying existing code

L — Liskov Substitution Principle

Subclasses should be substitutable for their parent.

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class Animal { void makeSound() => print('...'); }
class Dog extends Animal { @override void makeSound() => print('Woof'); }
void speak(Animal a) => a.makeSound(); // Works with Dog too

I — Interface Segregation Principle

Don't force clients to depend on interfaces they don't use.

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// ❌ Fat interface
abstract class Worker { void work(); void eat(); void sleep(); }

// ✅ Segregated
abstract class Workable { void work(); }
abstract class Eatable { void eat(); }
class HumanWorker implements Workable, Eatable { ... }
class RobotWorker implements Workable { void work() { ... } }

D — Dependency Inversion Principle

Depend on abstractions, not concretions.

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// ❌ Depends on concrete class
class OrderService { final SqlDatabase db = SqlDatabase(); }

// ✅ Depends on abstraction
abstract class Database { Future<void> save(Order order); }
class OrderService {
  final Database db; // Can be SQL, NoSQL, mock
  OrderService(this.db);
}

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2. DRY — Don't Repeat Yourself

Extract repeated code into reusable functions/widgets.

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// ✅ Reusable
Widget primaryButton(String text, VoidCallback onTap) =>
    ElevatedButton(onPressed: onTap, child: Text(text));

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3. KISS — Keep It Simple, Stupid

Write the simplest solution that works.

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4. YAGNI — You Aren't Gonna Need It

Don't add functionality until it's actually needed.

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5. Separation of Concerns

Each layer/module handles only its own responsibility (UI, Logic, Data).

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Summary Table