Coding Conventions and Detekt

Kotlin Coding Conventions

Follow these conventions to write Kotlin that other developers expect to read.

Naming

// Classes and interfaces: PascalCase
class UserRepository { }
interface Readable { }

// Functions and variables: camelCase
fun calculateTotal() { }
val userName = "Alice"

// Constants: SCREAMING_SNAKE_CASE or camelCase
const val MAX_RETRIES = 3
const val defaultPort = 8080

// Packages: lowercase, no underscores
package com.example.userservice

Prefer val

// Good — read-only, safe
val users = listOf("Alice", "Bob")

// Avoid unless you need to change it
var count = 0

Use Expression Bodies for Short Functions

// Good — clean one-liner
fun isAdult(age: Int): Boolean = age >= 18

// Fine for longer functions
fun process(data: String): String {
    val cleaned = data.trim()
    val upper = cleaned.uppercase()
    return upper
}

Use Data Classes for Data Holders

// Good — auto-generates equals, hashCode, toString, copy
data class Product(val id: Int, val name: String, val price: Double)

// Avoid — manual boilerplate when a data class works
class Product(val id: Int, val name: String, val price: Double) {
    override fun equals(other: Any?): Boolean { /* ... */ }
    override fun hashCode(): Int { /* ... */ }
}

Use Scope Functions to Reduce Boilerplate

// Configure with apply
val server = Server().apply {
    host = "localhost"
    port = 8080
    timeout = 30
}

// Null-safe transformation with let
val email: String? = getEmail()
val domain = email?.let { it.substringAfter("@") }

Detekt — Static Analysis

Detekt finds code smells, style violations, and potential bugs.

Install

Add to your build.gradle.kts:

plugins {
    kotlin("jvm") version "2.4.10"
    id("io.gitlab.arturbosch.detekt") version "1.23.8"
}

repositories {
    mavenCentral()
}

Run

./gradlew detekt

Detekt checks your code and prints issues:

src/main/kotlin/UserService.kt:15:1
ComplexMethod [Complexity: 12] — method doEverything is too complex

Configure

Create config/detekt.yml to customize rules:

complexity:
  LongMethod:
    threshold: 60         # max lines per function (default: 60)
  LongParameterList:
    functionThreshold: 6  # max params per function (default: 6)
  ComplexCondition:
    threshold: 4          # max conditions in one if (default: 4)

style:
  MaxLineLength:
    maxLineLength: 120    # max characters per line
  WildcardImport:
    active: true          # ban import com.example.*

Run with the config:

./gradlew detekt --config config/detekt.yml

Common Detekt Rules

Suppress Rules When Needed

@Suppress("MagicNumber")
fun calculateFibonacci(n: Int): Int {
    if (n <= 1) return n       // MagicNumber suppressed
    return calculateFibonacci(n - 1) + calculateFibonacci(n - 2)
}

ktlint — Format Checker

Detekt finds code smells. ktlint checks formatting (spacing, indentation, imports). Use both together:

plugins {
    id("org.jlleitschuh.gradle.ktlint") version "12.3.0"
}

Auto-format your code:

./gradlew ktlintFormat

CI Integration

Add detekt to your CI pipeline (GitHub Actions, GitLab CI) to catch issues before merge:

# .github/workflows/ci.yml
jobs:
  check:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-java@v4
        with:
          java-version: '17'
          distribution: 'temurin'
      - run: ./gradlew detekt test

The build fails if Detekt finds issues or tests fail.

Practice Checklist

Apply these to your code:

• All variables use val unless mutation is needed
• Data classes hold data, regular classes have behavior
• No magic numbers — use const val or named parameters
• Functions are under 30 lines
• No wildcard imports (import x.*)
• Run ./gradlew detekt with zero issues
• Run ./gradlew ktlintFormat to auto-fix style