We can create singleton to group related utility functions into a single entity. Suppose we define multiple functions related to calendar :

• isLeapYear(year: Int): Boolean
• getNoOfDaysInMonth(month: Int, year: Int): Int
• getNoOfDaysBetween(start: Date, end: Date): Int

Instead of defining them directly in a file, we can define them in a singleton called CalendarUtils :

 object CalendarUtils {

    fun isLeapYear(year: Int): Boolean { TODO() }

    fun getNoOfDaysInMonth(month: Int, year: Int): Int { TODO() }

    fun getNoOfDaysBetween(start: Date, end: Date): Int { TODO() }
}
 

We can directly access this functions using the singleton name :

 object CalendarUtils {

    fun isLeapYear(year: Int): Boolean { TODO() }

    fun getNoOfDaysInMonth(month: Int, year: Int): Int { TODO() }

    fun getNoOfDaysBetween(start: Date, end: Date): Int { TODO() }
}

fun main() {
		val is2020ALeapYear = CalendarUtils.isLeapYear(2020)
		val noOfDaysInFeb2023 = CalendarUtils.getNoOfDaysInMonth(2, 2023)
}
 

We can create singleton to group multiple constants. Constant is a variable with fixed value used all across an application. For example, in a game we may have constants like

• maximum lifelines,
• total number of levels,
• maximum number of players etc.

We can define them in a singleton called Constants :

 object Constants {
    const val MAX_LIFELINES = 20
    const val TOTAL_LEVELS = 100
    const val MAX_PLAYERS = 6
}
 

We can access them directly as :

 if (level == Constants.TOTAL_LEVELS) {
    println("Congratulations, you have completed the game!")
}
 

Note :

• Singletons don’t have constructors. They are constructed as soon as the program runs and reside in memory till the end.
• Like normal classes, singetons can also inherit from open classes.
• Singletons can’t be declared as open i.e. they can’t be inherited.