Kotlin: Lists of Weekday and Weekend Abbreviations

listOf("Mon", "Tue", "Wed", "Thu", "Fri") and listOf("Sat", "Sun"). The simplest puzzle in the series — useful for showing how Copilot reads paired comments.

This one's tiny: hardcode two lists. The interesting part is what Copilot infers from comment context — and how to do this correctly with java.time.DayOfWeek instead of hardcoded strings.

The Copilot prompt

// Create a list of 3-letter abbreviations for weekdays
val weekdays =

Copilot completes:

val weekdays = listOf("Mon", "Tue", "Wed", "Thu", "Fri")

Then continue:

// Create a list of 3-letter abbreviations for weekend days
val weekends =

Copilot generates:

val weekends = listOf("Sat", "Sun")

println("Weekdays: $weekdays")
println("Weekends: $weekends")

Walkthrough

listOf(...) is the standard immutable-list constructor. Five strings for weekdays, two for weekends. Done.

But hardcoded strings are fragile. They: - Break for non-English locales. - Don't match java.time.DayOfWeek enum names. - Have to be re-typed for "Monday" full names, "M" single-letter, etc.

The richer answer uses java.time.

With java.time

import java.time.DayOfWeek
import java.time.format.TextStyle
import java.util.Locale

val (weekdays, weekends) = DayOfWeek.values().partition { it.value <= 5 }

val weekdayAbbrevs = weekdays.map { it.getDisplayName(TextStyle.SHORT, Locale.ENGLISH) }
val weekendAbbrevs = weekends.map { it.getDisplayName(TextStyle.SHORT, Locale.ENGLISH) }

println(weekdayAbbrevs)   // [Mon, Tue, Wed, Thu, Fri]
println(weekendAbbrevs)   // [Sat, Sun]

Three steps:

1. Partition by ISO weekday. ISO defines Monday=1, Sunday=7. Days 1-5 are weekdays; 6-7 are weekend.
2. Map to localized short name. getDisplayName(TextStyle.SHORT, locale) produces "Mon", "Tue", etc. for English.
3. Use the result. Now they adapt to locale.

For French:

val french = DayOfWeek.MONDAY.getDisplayName(TextStyle.SHORT, Locale.FRENCH)
// "lun." (with trailing period — French convention)

For Japanese:

val japanese = DayOfWeek.MONDAY.getDisplayName(TextStyle.SHORT, Locale.JAPANESE)
// "月" (single character)

TextStyle options

TextStyle.FULL — "Monday" TextStyle.SHORT — "Mon" TextStyle.NARROW — "M"

There's also FULL_STANDALONE, SHORT_STANDALONE, NARROW_STANDALONE — used in some languages where the day name differs based on context (e.g., Slavic languages have nominative vs. genitive forms).

When hardcoded strings are fine

For small one-off scripts in English, listOf("Mon", "Tue", ...) is fine. The bug emerges only when the app is translated.

For anything user-facing — labels, calendar headers, scheduling UIs — always go through the locale-aware API. It's roughly the same amount of code, and it scales to every locale Java supports.

Filtering by weekday/weekend

import java.time.LocalDate

val today = LocalDate.now()
val isWeekend = today.dayOfWeek.value >= 6
println(if (isWeekend) "Weekend" else "Weekday")

// Or via a Set
val weekend = setOf(DayOfWeek.SATURDAY, DayOfWeek.SUNDAY)
val isWeekend2 = today.dayOfWeek in weekend

Set membership is more readable for "is this in the weekend group."

All days in order

val allDays = DayOfWeek.values().toList()
println(allDays)
// [MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY]

DayOfWeek.values() returns an Array<DayOfWeek> in ISO order. .toList() for a List.

For weeks-start-on-Sunday (US convention), reorder:

val usOrder = listOf(DayOfWeek.SUNDAY) + DayOfWeek.values().toList().dropLast(1)
// [SUNDAY, MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY]

The reorder is locale-specific — WeekFields.firstDayOfWeek from java.time.temporal.WeekFields gives you the first day for a locale.

Common mistakes

Hardcoded for one locale. Breaks the moment users see another language.

Wrong abbreviations. "Tu" vs "Tue" — both seen in the wild. getDisplayName picks the official locale convention.

Mixing ISO and Apple weekday numbers. ISO: Monday=1. Apple: Sunday=1. Don't reuse magic numbers across systems.

Forgetting value <= 5 for weekday. Saturday is 6, Sunday is 7. Inclusive <= matters.

Treating 5 days as universal. Many cultures have 6-day workweeks; some have Friday-Saturday weekends (Middle East). For real schedule UIs, get the rule from a source — don't bake it in.

What's next

Episode 20: Tell Copilot to "secure" a Jetpack Compose app. A meta-puzzle on what "secure" means and what Copilot will produce when prompted vaguely.

Recap

listOf("Mon", ...) works for one-off scripts. For real apps, use DayOfWeek.values() + getDisplayName(TextStyle.SHORT, locale) to get locale-aware abbreviations. ISO ordering: Monday=1, Sunday=7; weekend is 6-7. TextStyle.FULL/SHORT/NARROW for full name / 3-letter / 1-letter forms.

Next episode: securing a Compose app.