【Kotlin/Android】Calendarクラスの使い方！日付の比較や操作、曜日の取得方法

import java.util.Calendar

// 現在の日付と時刻を保持するインスタンスを取得
val c = Calendar.getInstance()

java.util.GregorianCalendar[time=1714824725299,areFieldsSet=false,areAllFieldsSet=false,lenient=true,zone=sun.util.calendar.ZoneInfo[id="Asia/Tokyo",offset=32400000,dstSavings=0,useDaylight=false,transitions=10,lastRule=null],firstDayOfWeek=1,minimalDaysInFirstWeek=1,ERA=1,YEAR=2024,MONTH=4,WEEK_OF_YEAR=18,WEEK_OF_MONTH=1,DAY_OF_MONTH=4,DAY_OF_YEAR=125,DAY_OF_WEEK=7,DAY_OF_WEEK_IN_MONTH=1,AM_PM=1,HOUR=0,HOUR_OF_DAY=12,MINUTE=12,SECOND=5,MILLISECOND=299,ZONE_OFFSET=0,DST_OFFSET=0]

val c = Calendar.getInstance()
val year = c.get(Calendar.YEAR)
val month = c.get(Calendar.MONTH)
val day = c.get(Calendar.DAY_OF_MONTH)
val hour = c.get(Calendar.HOUR_OF_DAY)
val minute = c.get(Calendar.MINUTE)
val second = c.get(Calendar.SECOND)

print(year) // 2024
print(month + 1 ) // 5
print(day) // 9
print(hour) // 21
print(minute) // 10
print(second) // 17

val c = Calendar.getInstance()

val timeZone = TimeZone.getTimeZone("Asia/Tokyo") // 例: 日本時間
c.timeZone = timeZone

val year = c.get(Calendar.YEAR)
val month = c.get(Calendar.MONTH)
val day = c.get(Calendar.DAY_OF_MONTH)
val hour = c.get(Calendar.HOUR_OF_DAY)
val minute = c.get(Calendar.MINUTE)
val second = c.get(Calendar.SECOND)

print(year) // 2024
print(month + 1 ) // 5
print(day) // 9
print(hour) // 21
print(minute) // 10
print(second) // 17

val timeZone = TimeZone.getTimeZone("UTC") // 例: UTCに変更
c.timeZone = timeZone

val year = c.get(Calendar.YEAR)
val month = c.get(Calendar.MONTH)
val day = c.get(Calendar.DAY_OF_MONTH)
val hour = c.get(Calendar.HOUR_OF_DAY)
val minute = c.get(Calendar.MINUTE)
val second = c.get(Calendar.SECOND)

print(year) // 2024
print(month + 1 ) // 5
print(day) // 9
print(hour) // 12
print(minute) // 10
print(second) // 17

val availableTimeZones = TimeZone.getAvailableIDs()
availableTimeZones.forEach { println(it) }

val c = Calendar.getInstance()
// 曜日の取得
val dayOfWeek = c.get(Calendar.DAY_OF_WEEK)

// DAY_OF_WEEK の値から曜日を取得
val dayOfWeekString = when (dayOfWeek) {
    Calendar.SUNDAY -> "日曜日"
    Calendar.MONDAY -> "月曜日"
    Calendar.TUESDAY -> "火曜日"
    Calendar.WEDNESDAY -> "水曜日"
    Calendar.THURSDAY -> "木曜日"
    Calendar.FRIDAY -> "金曜日"
    Calendar.SATURDAY -> "土曜日"
    else -> "不明な曜日"
}

// 5日後
c.add(Calendar.DATE, 5)

// 5時間前
c.add(Calendar.HOUR, -5)

val date = Date()
val c = Calendar.getInstance()
c.time = date

val calendar1 = Calendar.getInstance()
val calendar2 = Calendar.getInstance()

// calendar1がcalendar2よりも後の場合
if (calendar1.after(calendar2)) {
    println("calendar1はcalendar2よりも後です。")
}

// calendar1がcalendar2よりも前の場合
if (calendar1.before(calendar2)) {
    println("calendar1はcalendar2よりも前です。")
}

// calendar1とcalendar2が同じ日付の場合
if (calendar1.compareTo(calendar2) == 0) {
    println("calendar1とcalendar2は同じ日付です。")
}