kotlin-best-practices

Purpose

Apply idiomatic Kotlin patterns and best practices from official JetBrains documentation.

Core Principles

Concise yet readable
Null safety by design
Interoperability with Java
Functional when appropriate

Rules

1. Naming Conventions

packages:
  style: lowercase, no underscores
  example: org.example.project

classes_objects:
  style: UpperCamelCase
  example: DeclarationProcessor

functions_variables:
  style: lowerCamelCase
  example: processDeclarations, declarationCount

constants:
  style: SCREAMING_SNAKE_CASE
  example: const val MAX_COUNT = 8

backing_properties:
  style: underscore prefix
  example: private val _elementList

acronyms:
  two_letters: both uppercase (IOStream)
  three_plus: capitalize first only (XmlFormatter)

2. Source File Organization

file_naming:
  single_class: name after the class (MyClass.kt)
  multiple_classes: descriptive UpperCamelCase (ProcessDeclarations.kt)
  platform_specific: add suffix (Platform.jvm.kt)

class_layout:
  1: Property declarations and initializer blocks
  2: Secondary constructors
  3: Method declarations
  4: Companion object

directory_structure:
  - Follow package structure with common root omitted
  - Example: org.example.kotlin.network → network/

3. Formatting

indentation:
  - 4 spaces (no tabs)
  - Opening brace at end of line
  - Closing brace on separate line

horizontal_whitespace:
  around_binary_operators: "a + b"
  no_around_range: "0..i"
  no_around_unary: "a++"
  after_control_keywords: "if (condition)"
  no_before_parentheses: "method()"
  never_around_dot: "foo.bar()"

colons:
  type_supertype: "class Foo : Bar"
  declaration_type: "val x: Int"
  always_space_after: true

trailing_commas:
  recommended: true
  reason: cleaner diffs, easier reordering

4. Functions

expression_bodies:
  prefer: "fun foo() = 1"
  over: |
    fun foo(): Int {
        return 1
    }

default_parameters:
  prefer: "fun foo(a: String = \"a\")"
  over: overloaded functions

single_line_signatures:
  if_fits: "fun foo(a: Int): String = ..."
  otherwise: |
    fun longMethodName(
        argument: ArgumentType = defaultValue,
        argument2: AnotherArgumentType,
    ): ReturnType { }

unit_return:
  avoid: "fun foo(): Unit { }"
  prefer: "fun foo() { }"

5. Null Safety

principles:
  - Prefer non-null types by default
  - Use ? only when nullability is meaningful
  - Leverage safe calls (?.) and elvis (?:)

patterns: |
  // Safe call
  val length = text?.length

  // Elvis operator
  val name = user?.name ?: "Anonymous"

  // Let for null checks
  user?.let {
      println(it.name)
  }

  // Not-null assertion (use sparingly)
  val name = user!!.name

6. Idiomatic Patterns

immutability:
  prefer: "val over var"
  collections: "listOf() over arrayListOf()"

conditionals:
  binary: use if
  multiple: use when

  patterns: |
    // if expression
    return if (x) foo() else bar()

    // when expression
    return when(x) {
        0 -> "zero"
        else -> "nonzero"
    }

functional:
  prefer: "list.filter { it > 10 }.map { it * 2 }"
  over: manual loops (except forEach)

ranges:
  prefer: "for (i in 0..<n)"
  avoid: "for (i in 0..n - 1)"

type_aliases:
  use_for: |
    typealias MouseClickHandler = (Any, MouseEvent) -> Unit
    typealias PersonIndex = Map<String, Person>

7. Lambdas

formatting:
  - Spaces around curly braces: "list.filter { it > 10 }"
  - Single lambda outside parentheses
  - it for single parameter

patterns: |
  // Outside parentheses
  list.filter { it > 10 }

  // Multiple parameters
  list.fold(0) { acc, item -> acc + item }

  // Destructuring
  map.forEach { (key, value) -> println("$key = $value") }

8. Coroutines

principles:
  - Use suspend functions for async operations
  - Structured concurrency with scopes
  - Proper exception handling

patterns: |
  // Suspend function
  suspend fun fetchData(): Data {
      return withContext(Dispatchers.IO) {
          api.getData()
      }
  }

  // Coroutine scope
  lifecycleScope.launch {
      val data = fetchData()
      updateUI(data)
  }

  // Exception handling
  try {
      val result = async { fetchData() }.await()
  } catch (e: Exception) {
      handleError(e)
  }

9. Documentation

format: |
  /**
   * Returns the absolute value of the given [number].
   */
  fun abs(number: Int): Int { ... }

best_practices:
  - Incorporate parameters into description
  - Use brackets for parameter references
  - Use @param/@return only for lengthy descriptions

Application

When writing or reviewing Kotlin code:

1. Always prefer val over var
2. Always use null-safe types appropriately
3. Prefer expression bodies for simple functions
4. Prefer functional operations over loops
5. Use default parameters over overloads
6. Use when for multiple conditions
7. Leverage extension functions
8. Document public APIs