Kotlin by Example: Multiple return values
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Kotlin does not have built-in support for returning multiple values. However, it is relatively easy to define dataclasses which let you return multiple values. There are two such dataclasses in the standard library. |
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For pairs, there is the |
fun firstAndLastName(): Pair<String, String> {
return Pair("Ruben", "van Nieuwpoort");
}
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Likewise, for triples there is the |
fun allMyNames(): Triple<String, String, String> {
return Triple("Ruben", "van", "Nieuwpoort");
}
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There is no |
data class Quadruple<out A, out B, out C, out D>(
val first: A,
val second: B,
val third: C,
val fourth: D
)
fun evenMoreNames(): Quadruple<String, String,
String, String> {
return Quadruple("Fernando", "de", "los", "Santos");
}
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Yet another solution is to return a list. |
fun ridiculouslyLongName(): List<String> {
return listOf("Adolph", "Blaine", "Charles", "David")
}
fun main() {
val names = firstAndLastName()
println(names.first + " " + names.second)
val moreNames = allMyNames()
println(moreNames.first + " " +
moreNames.second + " " +
moreNames.third)
val manyNames = evenMoreNames()
println(manyNames.first + " " +
manyNames.second + " " +
manyNames.third + " " +
manyNames.fourth)
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It is also possible to do a destructuring declaration, which "destructures" the values into separate variables. |
val (firstName, secondName, thirdName) = allMyNames()
println(firstName)
println(secondName)
println(thirdName)
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Destructuring declarations also work for functions that return lists. |
val (name1, name2) = ridiculouslyLongName()
println(name1)
println(name2)
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Destructuring declarations are just syntactic sugar. This is the desugared version of the last paragraph of code. |
val temp = ridiculouslyLongName()
val desugaredName1 = temp.component1()
val desugaredName2 = temp.component2()
println(name1)
println(name2)
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There can be up to 5 variables in a destructuring declaration. The number of variables can not be bigger than the number of values that the function returns. |
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For types with a fixed number of values the number of values is checked at compile time. For collection types, an |
val (n1, n2, n3, n4, n5) = ridiculouslyLongName()
}
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$ kotlinc main.kt -include-runtime -d main.jar
$ java -jar Main.jar
Ruben van Nieuwpoort
Ruben van Nieuwpoort
Fernando de los Santos
Ruben
van
Nieuwpoort
Adolph
Blaine
Adolph
Blaine
Exception in thread "main" java.lang.ArrayIndexOutOfBounds
Exception: Index 4 out of bounds for length 4
at java.base/java.util.Arrays$ArrayList.get(Arrays
.java:4225)
at MainKt.main(main.kt:77)
at MainKt.main(main.kt)
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