Objects and companions in Kotlin

Singletons with object, statics with companion object.

// object declares a singleton; companion members ride on the class.
object TrailRegistry {
    private val open = mutableSetOf("Ridgeline", "Lakeshore")
    fun isOpen(name: String) = name in open
}

class Permit private constructor(val id: Int) {
    companion object {
        const val PREFIX = "KTL"
        fun issue(n: Int) = Permit(n)
    }
}

fun main() {
    println(TrailRegistry.isOpen("Lakeshore"))
    val permit = Permit.issue(207)
    println("${Permit.PREFIX}-${permit.id}")
}

How it works

1. object TrailRegistry declares and instantiates in one breath.
2. A companion object holds what other languages call static members.
3. private constructor plus Permit.issue funnels creation through a factory.

Keywords and builtins used here

• Int
• Permit
• String
• TrailRegistry
• class
• companion
• const
• constructor
• fun
• in
• isOpen
• issue
• main
• object
• private
• val

The run, in numbers

Lines

18

Characters to type

449

Tokens

109

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 101 seconds.

Type this snippet

Step 3 of 3 in Classes & objects, step 15 of 27 in Language basics.

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Objects and companions in other languages

• Scala