Pair in Kotlin

02 Aug, 2019
In programming, we call functions to perform a particular task. The best thing about function is that we can call it any number of times and it return some value after computation i.e. if we are having add() function then it always returns the sum of both the numbers entered.
But, functions has some limitations like function return only one value at a time. If there is need to return more than one value of different data type, then we can create a class and declare all the variables that we want to return from the function and after that create an object of the class and easily collect all the returned values in a list. The main problem is that if there are so many functions in the program which returns more than one value at a time then we have to create a separate class for all those functions and finally use that class. This process increases the verbosity and complexity of the program.
In order to deal with these type of problems, Kotlin introduced the concept of Pair and Triple.
Kotlin language provides a simple datatype to store two values in a single instance. This can be done using a data class known as Pair. It is a simple generic class that can store two values of same or different data types, and there can or can not be a relationship between the two values. The comparison between two Pair objects is done on the basis of values, i.e. two Pair objects are equal if their values are equal.
Class Definition –
data class Pair<out A, out B> : Serializable

There are two parameters:A – type of the first valueB – type of the second value
In Kotlin, constructor is a special member function that is invoked when an object of the class is created primarily to initialize variables or properties. To create a new instance of the Pair we use:
Pair(first: A, second: B)
Kotlin example of creating pair using the constructor –
fun main() {    val (x, y) = Pair(1, "Geeks")    println(x)    println(y)}
Output:
1
Geeks

We can either receive the values of pair in a single variable or we can use first and second properties to extract the values.first: This field stores the first value of the Pair.second: This field stores the second value of the Pair.
Kotlin program to retrieve the values of Pair using properties –
fun main() {    // declare pair    var pair = Pair("Hello Geeks", "This is Kotlin tutorial")    println(pair.first)    println(pair.second)}
Output:
Hello Geeks
This is Kotlin tutorial

toString(): This function returns the string equivalent of the Pair.
fun toString(): String

Kotlin program of using the function –
fun main() {    val obj = Pair(5,5)    println("String representation is "+obj.toString())    val pair = Pair("Geeks", listOf("Praveen", "Gaurav", "Abhi"))    print("Another string representation is "+pair.toString())}
Output:
String representation is (5, 5)
Another string representation is (Geeks, [Praveen, Gaurav, Abhi])

As we have learnt in previous articles, extension functions gives ability to add more functionality to the existing classes, by without inheriting them.toList(): This function returns the List equivalent of the given Pair.
fun <T>Pair<T, T>.toList(): List

Kotlin program of using the extended function –
fun main() {    // first pair    var obj = Pair(1,2)    val list1: List<Any> = obj.toList()    println(list1)    // second pair    var obj2 = Pair("Hello","Geeks")    val list2: List<Any> = obj2.toList()    println(list2)}
Output
[1, 2]
[Hello, Geeks]

Kotlin
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