Kotlin is a modern programming language that runs on the Java Virtual Machine. It is a statically typed language that combines the best features of both object-oriented and functional programming.
Kotlin is a first-class citizen on the JVM, which means that it can be used to develop any kind of Java application, from simple command-line programs to complex web applications.
One of the most important features of Kotlin is that it supports higher-order functions. In this article, we will learn what higher-order functions are and how to use them in Kotlin.
A higher-order function is a function that takes one or more functions as arguments and returns a function as a result.
In Kotlin, higher-order functions are represented by the Function<T, R>
type. This type has two type parameters:
T
is the type of the argument to the function.R
is the type of the result of the function.The Function<T, R>
type can be used to represent any function that takes an argument of type T
and returns a result of type R
.
There are many built-in higher-order functions in Kotlin that are available for use. In this section, we will learn how to use some of the most commonly used higher-order functions.
map
FunctionThe map
function is a higher-order function that takes a function as an argument and applies it to each element of a list.
For example, suppose we have a list of strings and we want to convert them to uppercase. We can do this using the map
function:
val list = listOf("a", "b", "c")
val upperCaseList = list.map { it.toUpperCase() }
// upperCaseList is now a list of strings: ["A", "B", "C"]
In the example above, we have a list of strings and we use the map
function to apply the toUpperCase()
function to each element of the list.
The map
function can be used with any type of list, not just lists of strings. For example, we can use map
with a list of integers:
val list = listOf(1, 2, 3)
val doubleList = list.map { it * 2 }
// doubleList is now a list of integers: [2, 4, 6]
filter
FunctionThe filter
function is a higher-order function that takes a function as an argument and applies it to each element of a list. The function should return true
or false
.
The filter
function will return a new list that contains only the elements for which the function returns true
.
For example, suppose we have a list of strings and we want to filter out all the strings that are longer than three characters. We can do this using the filter
function:
val list = listOf("a", "ab", "abc", "abcd")
val filteredList = list.filter { it.length <= 3 }
// filteredList is now a list of strings: ["a", "ab", "abc"]
In the example above, we have a list of strings and we use the filter
function to remove all the strings that are longer than three characters.
The filter
function can be used with any type of list, not just lists of strings. For example, we can use filter
with a list of integers:
val list = listOf(1, 2, 3, 4, 5)
val filteredList = list.filter { it % 2 == 0 }
// filteredList is now a list of integers: [2, 4]
In the example above, we have a list of integers and we use the filter
function to remove all the odd numbers.
reduce
FunctionThe reduce
function is a higher-order function that takes a function as an argument and applies it to each element of a list. The function should take two arguments and return a single value.
The reduce
function will return a single value that is the result of applying the function to each element of the list.
For example, suppose we have a list of integers and we want to find the sum of all the elements in the list. We can do this using the reduce
function:
val list = listOf(1, 2, 3, 4, 5)
val sum = list.reduce { a, b -> a + b }
// sum is now 15
In the example above, we have a list of integers and we use the reduce
function to find the sum of all the elements in the list.
The reduce
function can be used with any type of list, not just lists of integers. For example, we can use reduce
with a list of strings:
val list = listOf("a", "b", "c")
val concatenatedString = list.reduce { a, b -> a + b }
// concatenatedString is now "abc"
In the example above, we have a list of strings and we use the reduce
function to concatenate all the strings into a single string.
In this article, we have learned what higher-order functions are and how to use them in Kotlin. We have seen how to use the map
, filter
, and reduce
functions to transform and manipulate lists.
Higher-order functions are a powerful tool that can be used to write concise and readable code. If you are new to Kotlin, I encourage you to experiment with higher-order functions and see what you can come up with.