standards-kotlin

Kotlin Coding Standards

Core Principles

1. Explicitness: Explicit code over implicit magic
2. Readability: Readable code over clever tricks
3. Null Safety: Embrace Kotlin's null safety system
4. Immutability: Prefer val over var, immutable collections
5. Expressiveness: Use Kotlin's expressive features (data classes, sealed classes)
6. DRY: Don't Repeat Yourself - but keep it simple

General Rules

• Prefer val over var: Immutability by default
• Use data classes: For simple data holders
• Sealed classes/interfaces: For type-safe state modeling
• Early returns: Avoid deep nesting
• Descriptive names: Clear, meaningful names
• Minimal changes: Only change relevant code
• No over-engineering: Keep it simple
• Minimal comments: Self-explanatory code. Comments for "why", not "what"

Naming Conventions

Element	Convention	Example
Classes	PascalCase	UserService, OrderRepository
Interfaces	PascalCase	UserRepository, PaymentProcessor
Functions	camelCase	getUserById, calculateTotal
Properties	camelCase	firstName, totalAmount
Constants	UPPER_SNAKE_CASE	MAX_RETRY_COUNT, DEFAULT_TIMEOUT
Packages	lowercase.dot.separated	com.example.service, com.example.repository
Files	PascalCase.kt	UserService.kt, OrderRepository.kt
Test Classes	ClassNameTest	UserServiceTest, OrderRepositoryTest
Test Functions	backtick names	`should return user when id exists`

Project Structure

Gradle Kotlin DSL Project (Recommended)

myproject/
├── build.gradle.kts
├── settings.gradle.kts
├── gradle.properties
├── src/
│   ├── main/
│   │   ├── kotlin/
│   │   │   └── com/example/myapp/
│   │   │       ├── Application.kt          # Main entry point
│   │   │       ├── config/
│   │   │       │   └── AppConfig.kt        # Configuration
│   │   │       ├── domain/
│   │   │       │   └── User.kt             # Domain models
│   │   │       ├── repository/
│   │   │       │   └── UserRepository.kt   # Data access
│   │   │       ├── service/
│   │   │       │   └── UserService.kt      # Business logic
│   │   │       └── api/
│   │   │           └── UserController.kt   # REST endpoints
│   │   └── resources/
│   │       ├── application.conf
│   │       └── logback.xml
│   └── test/
│       ├── kotlin/
│       │   └── com/example/myapp/
│       │       ├── service/
│       │       │   └── UserServiceTest.kt
│       │       └── repository/
│       │           └── UserRepositoryTest.kt
│       └── resources/
│           └── application-test.conf
└── README.md