How do we return multiple values from a function i

问题:

How do I return 3 separate data values of the same type(Int) from a function in Kotlin?

I'm attempting to return the time of day, I need to return the Hour, Minute and Second as separate integers, but all in one go from the same function, is this possible?

In swift we do it like following,

func getTime() -> (Int, Int, Int) {
    ...
    return ( hour, minute, second)
}

can we achieve this in Kotlin?

P.S: I know I can use Array or Hashmap for this but I want to know if there exist something in kotlin like it is in swift.

回答1:

You can't create arbitrary tuples in Kotlin, instead, you can use data classes. One option is using the built in Pair and Triple classes that are generic and can hold two or three values, respectively. You can use these combined with destructuring declarations like this:

fun getPair() = Pair(1, "foo")

val (num, str) = getPair()

You can also destructure a List or Array, for up to the first 5 elements:

fun getList() = listOf(1, 2, 3, 4, 5)

val (a, b, c, d, e) = getList()

The most idiomatic way however would be to define your own data class, which allows you to return a meaningful type from your function:

data class Time(val hour: Int, val minute: Int, val second: Int)

fun getTime(): Time {
    ...
    return Time(hour, minute, second)
}

val (hour, minute, second) = getTime()

回答2:

According to the Kotlin documentation (https://kotlinlang.org/docs/reference/multi-declarations.html#example-returning-two-values-from-a-function) you can achieve it like that:

data class TimeData(val hour: Int, val minute: Int, val second: Int)

fun getTime(): TimeData {
    // do your calculations

    return TimeData(hour, minute, second)
}

// Get the time.
val (hour, minute, second) = getTime()

回答3:

you can return multiple values like shown below

data class Result(val result: Int, val status: Status)
fun function(...): Result {
    // computations

    return Result(result, status)
}

// Now, to use this function:
val (result, status) = function(...)

see this documentation

for more example see this link

回答4:

There is no tuple in Kotlin. Alternatively, you can use data class with destructuring declaration.

data class Time(val hour: Int, val minute: Int, val second: Int)

func getTime(): Time {
    ...
    return Time(hour, minute, second)
}

//Usage
val time = getTime()
println("${time.hour}:${time.minute}:${time.second}")
//Or
val (hour, minute, second) = getTime()
println("${hour}:${minute}:${second}")

If you don't want to create data class for each specific case, you may create some generic data classes and use typealias for clarity.

data class Two<A, B>(val a: A, val b: B)
data class Three<A, B, C>(val a: A, val b: B, val c: C)
data class Four<A, B, C, D>(val a: A, val b: B, val c: C, val d: D)
...

typealias Time = Three<Int, Int, Int>

But, obviously, the disadvantage is that you must make use of destructuring declaration in order to give it a proper property name.

val time = getTime()
println("${time.a}:${time.b}:${time.c}")

val (hour, minute, second) = getTime()    //destructuring declaration
println("${hour}:${minute}:${second}")

回答5:

It seems that you are aware of the obvious answer of creating a specific class to handle time. So I guess you are trying to avoid the small hassle of creating a class, or accessing each element of an array, etc. and are looking for the shortest solution in terms of extra code. I would suggest:

fun getTime(): Triple<Int, Int, Int> {
    ...
    return Triple( hour, minute, second)
}

and use it with deconstruction:

var (a, b, c) = getTime()

If you need 4 or 5 return values (you cannot deconstruct more than 5), go with Array:

fun getTime(): Array<Int> {
    ...
    return arrayOf( hour, minute, second, milisec)
}

and

var (a, b, c, d) = getTime()

P.S.: you can use less variables than there are values when deconstructing, like var (a, b) = getTime() but you cannot use more or you'll get an ArrayIndexOutOfBoundsException