Kotlin by Example: Object declarations
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Many programming languages support |
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For functions that do not need state, a top-level function can be used. |
fun greet() {
println("Hello!")
}
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For functions that do need state, we can use an object declaration, which essentially creates a singleton object. An object declaration can use interfaces or inheritance, just like a normal class. |
object Utils {
private var n = 0
fun getUniqueNumber(): Int {
return n++
}
}
class Counter() {
private var internalCounter = 0
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Functions in an object declaration can not access private or protected members of their arguments. To do this, a companion object can be used. |
companion object {
fun getNumber(c: Counter): Int {
return c.internalCounter++
}
}
}
fun main() {
greet()
println(Utils.getUniqueNumber())
println(Utils.getUniqueNumber())
println(Utils.getUniqueNumber())
val c = Counter()
println(Counter.getNumber(c))
println(Counter.getNumber(c))
println(Counter.getNumber(c))
}
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$ kotlinc main.kt -include-runtime -d main.jar
$ java -jar Main.jar
Hello!
0
1
2
0
1
2
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