best-practices.md

Use A rather than B for simplicity or clarity

A	B	Note
with(...)	run(...)	if the receiver is not nullable.
emptyList emptySet emptyMap emptyFlow	listOf setOf mapOf flowOf
buildList buildSet buildMap	mutableListOf mutableSetOf mutableMapOf
repeat(n) { println(it) }	for (i in 0..<n) { println(i) }
Iterable<T>.find(...)	Iterable<T>.firstOrNull(...)	find is an alias of firstOrNull.
Collection<T>.size String.length	Collection<T>.count() String.count()
String.substring(...)	String.take(...)
Set<T>	List<T>	if elements are unique and unordered. Don't overuse List<T>.
SortedSet<T>	Set<T>	if elements must remain sorted.
SortedMap<T>	Map<T>	if keys must remain sorted.
Iterable<T>.forEachIndexed { i, x -> ... }	Iterable<T>.forEach { ... }	if you need to access both indices and elements.
nullableString.orEmpty() nullableList.orEmpty()	nullableString ?: "" nullableList ?: emptyList()
CharSequence.lastIndex List<T>.lastIndex	String.length - 1 List<T>.size - 1
List<Int>.sumBy {...} or List<Int>.sumByDouble {...}	List<Int>.map {...}.sum()
(this.)javaClass.simpleName	this::class.java.simpleName
Flow.filterNotNull()	Flow.flatMapConcat { if (it != null) flowOf(it) else emptyFlow() }

Prefer infix notation

Recommended	Not recommended
in !in	contains
A to B	Pair(A, B)

Date and time

Recommended	Not recommended
kotlin.time.Duration	java.time.Duration
DateTimeFormatter.ISO_LOCAL_DATE	DateTimeFormatter.ofPattern("yyyy-MM-dd")
DateTimeFormatter.ISO_LOCAL_TIME	DateTimeFormatter.ofPattern("HH:mm:ss")

Use onEach or onEachIndexed when you want to perform a side effect for clarity

For example, the following is better ...

val capitals = listOf('a', 'b', 'c')
        .onEach { println(it) } // a b c
        .map { it.uppercaseChar() }

... than the following.

val capitals = listOf('a', 'b', 'c')
        .map {
            println(it) // a b c
            it.uppercaseChar()
        }

Misc

• Decompose a pair if possible.
  • Recommended: listOf("apple" to "🍎", "orange" to "🍊").map { it.first + it.second })
  • Not recommended: listOf("apple" to "🍎", "orange" to "🍊").map { (fruit, emoji) -> fruit + emoji }
• When defining a function, make the parameter types as abstract as possible.
  • e.g. Use a type as left as possible.
    • Iterable > Collection > (Set) > List.
• Mark a function with suspend if you need to call another suspend function in it, but cannot access CoroutineScope.