Why Kotlin?

Concise

Drastically reduce the amount of boilerplate code

/*

Create a POJO with getters, setters, `equals()`, `hashCode()`, `toString()` and `copy()` in a single line:

*/

data class Customer(val name: String, val email: String, val company: String)

// Or filter a list using a lambda expression:

val positiveNumbers = list.filter { it > 0 }

// Want a singleton? Create an object:

object ThisIsASingleton {

val companyName: String = "JetBrains"

}

Safe

Avoid entire classes of errors such as null pointer exceptions

/*

Get rid of those pesky NullPointerExceptions, you know, The Billion Dollar Mistake

*/

var output: String

output = null   // Compilation error

// Kotlin protects you from mistakenly operating on nullable types

val name: String? = null    // Nullable type

println(name.length())      // Compilation error

// And if you check a type is right, the compiler will auto-cast it for you

fun calculateTotal(obj: Any) {

if (obj is Invoice)

obj.calculateTotal()

}

Interoperable

Leverage existing libraries for the JVM, Android, and the browser

/*

Use any existing library on the JVM, as there’s 100% compatibility, including SAM support.

*/

import io.reactivex.Flowable

import io.reactivex.schedulers.Schedulers

Flowable

.fromCallable {

Thread.sleep(1000) //  imitate expensive computation

"Done"

}

.subscribeOn(Schedulers.io())

.observeOn(Schedulers.single())

.subscribe(::println, Throwable::printStackTrace)

// Target either the JVM or JavaScript. Write code in Kotlin and decide where you want to deploy to

import kotlin.browser.window

fun onLoad() {

window.document.body!!.innerHTML += "<br/>Hello, Kotlin!"

}

Tool-friendly

Choose any Java IDE or build from the command line

---

Higher-Order Functions

A higher-order function is a function that takes another function as parameter and/or returns a function.

fun calculate(x: Int, y: Int, operation: (Int, Int) -> Int): Int {  // 1

return operation(x, y)                                          // 2

}

fun sum(x: Int, y: Int) = x + y                                     // 3

fun main() {

val sumResult = calculate(4, 5, ::sum)                          // 4

val mulResult = calculate(4, 5) { a, b -> a * b }               // 5

println("sumResult $sumResult, mulResult $mulResult")

}

Output：

sumResult 9, mulResult 20

1. Declares a higher-order function. It takes two integer parameters, ​​x​​​ and ​​y​​​. Additionally, it takes another function ​​operation​​​ as a parameter. The ​​operation​​ parameters and return type are also defined in the declaration.
2. The higher order function returns the result of ​​operation​​ invocation with the supplied arguments.
3. Declares a function that matches the ​​operation​​signature.
4. Invokes the higher-order function passing in two integer values and the function argument ​​::sum​​​. ​​::​​ is the notation that references a function by name in Kotlin.
5. Invokes the higher-order function passing in a lambda as a function argument. Looks clearer, doesn't it?