Unlock the Power of Lambda Expressions in Kotlin

What is a Lambda Expression?

Imagine a function without a name, passed as an expression without declaration. That’s what a lambda expression is – an anonymous function that can be used immediately.

val greeting = { println("Hello, World!") }
greeting()

This expression doesn’t accept any parameters and doesn’t return any value.

Lambda Expressions with Parameters and Return Types

But what if you need a lambda expression that takes parameters and returns a value? No problem!

val multiply = { a: Int, b: Int -> a * b }
val result = multiply(2, 3)
println(result) // Output: 6

The output will be the result of this calculation.

The Role of Curly Braces

Notice how the lambda expression is enclosed inside curly braces. This is a key aspect of lambda expressions in Kotlin.

Higher-Order Functions: The Key to Functional Programming

Kotlin shines when it comes to functional programming. You can pass functions as arguments to other functions, and even return a function from another function. These are called higher-order functions. Lambda expressions are often passed to higher-order functions for convenience.

Passing Lambda Expressions to Functions

Let’s see how you can pass a lambda expression to a higher-order function.

fun callMe(lambda: () -> Unit) {
    lambda()
}

callMe { println("Hello from lambda!") }

The callMe() function takes a lambda expression as a parameter, which is then invoked without any parameters or return values.

Working with Collections Made Easy

Lambda expressions are frequently used when working with collections. Kotlin’s standard library provides several built-in functions that take lambdas to make your task easier. Let’s explore a couple of examples:

The maxBy() Function

The maxBy() function returns the first element yielding the largest value of the given function or null if there are no elements.

data class Person(val name: String, val age: Int)

val people = listOf(Person("Alice", 25), Person("Bob", 30), Person("Charlie", 20))
val oldestPerson = people.maxBy { it.age }
println(oldestPerson?.name) // Output: Bob

In this program, maxBy() takes a list of Person objects and returns the Person object with the maximum age.

The it Keyword: A Shortcut for Single Parameters

When a lambda expression accepts only one parameter, you can refer to the argument using the it keyword. This simplifies your code and makes it more readable.

val names = people.map { it.name }
println(names) // Output: [Alice, Bob, Charlie]

Combining maxBy() and startsWith() Functions

Now, let’s compute the maximum age of a Person object starting with the letter S.

val peopleStartingWithS = people.filter { it.name.startsWith("S") }
val oldestPersonStartingWithS = peopleStartingWithS.maxBy { it.age }
println(oldestPersonStartingWithS?.name) // Output: Sam (assuming there is a Person named Sam)

We’ll use the maxBy() and startsWith() functions to achieve this. The startsWith() function returns true if the string starts with the specified character passed as an argument.