Kotlin

Name

The name is derived from Kotlin Island, a Russian island in the Gulf of Finland, near Saint Petersburg. Andrey Breslav, Kotlin's former lead designer, mentioned that the team decided to name it after an island, in imitation of the Java programming language, which shares a name with the Indonesian island of Java.^[11]

Development

The first commit to the Kotlin Git repository was on 8 November 2010.^[12]

In July 2011, JetBrains unveiled Project Kotlin, a new JVM language that had been under development for a year.^[13] JetBrains lead Dmitry Jemerov said that most languages lacked the features they were looking for, except for Scala. However, he cited Scala's slow compilation time as a deficiency.^[13] One of Kotlin's stated goals is to compile as quickly as Java. In February 2012, JetBrains open-sourced the project under the Apache 2 license.^[14]

JetBrains expected Kotlin to drive sales of IntelliJ IDEA.^[15]

Kotlin 1.0 was released on 15 February 2016,^[16] which is considered the first officially stable release, and JetBrains has committed to long-term backwards compatibility starting with this version.

At Google I/O 2017, Google announced first-class support for Kotlin on Android.^[17] On 7 May 2019, Google announced that Kotlin is now its preferred language for Android app developers.^[7]

Procedural programming style

Kotlin relaxes the Java restriction on static methods and variables, allowing them to exist outside a class body. Static objects and functions can be defined at the top level of the package without requiring a redundant class-level scope. For compatibility with Java, Kotlin provides the JvmName annotation, which specifies the class name used when the package is viewed from a Java project. For example, @file:JvmName("JavaClassName").

Main entry point

As in C, C++, C#, Java, and Go, the entry point to a Kotlin program is a function named main, which may be passed an array containing any command-line arguments. This is optional since Kotlin 1.3.^[26] Perl, PHP, and Unix shell–style string interpolation is supported. Type inference is also supported.

// Hello, World! example
fun main() {
    val scope = "World"
    println("Hello, $scope!")
}

fun main(args: Array<String>) {
    for (arg in args) {
        println(arg)
    }
}

Extension functions

Similar to C#, Kotlin allows adding an extension function to any class without the formalities of creating a derived class with new functions. An extension function has access to all the public interface of a class, which it can use to create a new function interface to a target class. An extension function will appear exactly like a class function and will be shown in code completion inspections of class functions. For example:

package org.wikipedia.examples

fun String.lastChar(): Char = get(length - 1)

println("Kotlin".lastChar()) // prints: n

By placing the preceding code in the top-level of a package, the String class is extended to include a lastChar function that was not included in the original definition of the String class.

// Overloading '+' operator using an extension function
operator fun Point.plus(other: Point): Point {
    return Point(x + other.x, y + other.y)
}

val p1 = Point(10, 20)
val p2 = Point(30, 40)
println(p1 + p2) // prints: Point(x=40, y=60)

Scope functions

Kotlin has five scope functions that allow changing the scope within the context of an object. The scope functions are let, run, with, apply, and also.^[27]

Unpack arguments with the spread operator

Similar to Python, the spread operator asterisk (*) unpacks an array's contents as individual arguments to a function, e.g.:

fun main(args: Array<String>) { 
    val list: List<String> = listOf("args: ", *args)
    println(list)
}

Destructuring declarations

Destructuring declarations decompose an object into multiple variables at once, e.g., a 2D coordinate object might be destructured into two integers, x and y.

For example, the Map.Entry object supports destructuring to simplify access to its key and value fields:

for ((key, value) in map) {
    println("$key: $value")
}

Nested functions

Kotlin allows local functions to be declared inside other functions or methods.

class User(val id: Int, val name: String, val address: String)
    
fun saveUserToDb(user: User) {
    fun validate(user: User, value: String, fieldName: String) {
        require(value.isNotEmpty()) {
            "Can't save user ${user.id}: empty $fieldName"
        }
    }
    
    validate(user, user.name, "Name") 
    validate(user, user.address, "Address")
    // Save user to the database 
    // ...
}

Classes are final by default

In Kotlin, to derive a new class from a base class, the base class must be explicitly marked as open; in contrast, most object-oriented languages, such as Java, are open by default.

Example of a base class that is open to deriving a new subclass from it:

// open on the class means this class will allow derived classes
open class MegaButton {

    // no-open on a function means that 
    //    polymorphic behavior disabled if function overridden in derived class
    fun disable() {
        // ...
    }

    // open on a function means that
    //    polymorphic behavior allowed if function is overridden in derived class
    open fun animate() {
        // ...
    }
}

class GigaButton: MegaButton() {

    // Explicit use of override keyword required to override a function in derived class
    override fun animate() {
        println("Giga Click!")
    }
}

Abstract classes are open by default

Abstract classes define abstract, or "pure virtual", placeholder functions that will be implemented in a derived class. Abstract classes are open by default.

// No need for the open keyword here, it's already open by default
abstract class Animated {

    // This virtual function is already open by default as well
    abstract fun animate()
  
    open fun stopAnimating() {
        // ...
    }

    fun animateTwice() {
        // ...
    }
}

Classes are public by default

Kotlin provides the following keywords to restrict visibility for top-level declarations (such as classes) and for class members: public, internal, protected, and private.

When applied to a class member:

When applied to a top-level declaration:

Example:

// Class is visible only to current module
internal open class TalkativeButton {
    // method is only visible to current class 
    private fun yell() = println("Hey!")
    // method is visible to current class and derived classes
    protected fun whisper() = println("Let's talk!")
}

internal class MyTalkativeButton: TalkativeButton() {
    fun utter() = super.whisper()
}
MyTalkativeButton().utter()

Primary constructor vs. secondary constructors

Kotlin supports specifying a "primary constructor" as part of the class definition itself, consisting of an argument list following the class name. This argument list supports an expanded syntax on Kotlin's standard function argument lists that enables declaration of class properties in the primary constructor, including visibility, extensibility, and mutability attributes. Additionally, when defining a subclass, properties in super-interfaces and super-classes can be overridden in the primary constructor.

// Example of class using primary constructor syntax
// (Only one constructor required for this class)
open class BaseUser(open var isSubscribed: Boolean)
open class PowerUser(protected val nickname: String, final override var isSubscribed: Boolean = true):BaseUser(isSubscribed) { }

However, when more than one constructor is needed for a class, a more general constructor can be defined using secondary constructor syntax, which closely resembles that of most object-oriented languages such as C++, C#, and Java.

// Example of class using secondary constructor syntax
// (more than one constructor required for this class)
class Context
class AttributeSet

open class View(ctx:Context) {
    constructor(ctx: Context, attr: AttributeSet): this(ctx)
}

class MyButton : View {
    // Constructor #1 
    constructor(ctx: Context) : super(ctx) {
        // ...
    }

    // Constructor #2
    constructor(ctx: Context, attr: AttributeSet) : super(ctx, attr) {
        // ... 
    }
}

Sealed classes

Sealed classes and interfaces restrict subclass hierarchies, meaning more control over the inheritance hierarchy.

Declaration of sealed interface and class:

sealed interface Expr
sealed class Job

All the subclasses of the sealed class are defined at compile time. No new subclasses can be added to it after the module containing the sealed class is compiled. For example, a sealed class in a compiled jar file cannot be subclassed.

sealed class Vehicle
data class Car(val brandName: String, val owner: String, val color: String): Vehicle()
class Bike(val brandName: String, val owner: String, val color: String): Vehicle()
class Tractor(val brandName: String, val owner: String, val color: String): Vehicle()

val kiaCar = Car("KIA", "John", "Blue")
val hyundaiCar = Car("Hyundai", "Britto", "Green")

Data classes

Kotlin's data class construct defines classes whose primary purpose is storing data, similar to Java's record types. Like Java's record types, the construct is similar to a regular class, except that the key methods equals, hashCode, and toString are automatically generated from the class's properties. Unlike Java's records, data classes are open for inheritance.

Kotlin interactive shell

$ kotlinc-jvm
type :help for help; :quit for quit
>>> 2 + 2
4
>>> println("Hello, World!")
Hello, World!

Kotlin as a scripting language

Kotlin can also be used as a scripting language. A script is a Kotlin source file using the .kts filename extension, with executable source code at the top-level scope:

// list_folders.kts
import java.io.File

val folders = File(args[0]).listFiles { file -> file.isDirectory() }
folders?.forEach(::println)

Scripts can be run by passing the -script option and the corresponding script file to the compiler.

$ kotlinc -script list_folders.kts "path_to_folder_to_inspect"

Null safety

Kotlin distinguishes between nullable and non-nullable data types. All nullable objects must be declared with a "?" postfix after the type name. Operations on nullable objects need special care from developers: a null-check must be performed before using the value, either explicitly, or with the aid of Kotlin's null-safe operators:

• ?. (the safe navigation operator) can be used to safely access a method or property of a possibly null object. If the object is null, the method will not be called, and the expression evaluates to null. Example:

// returns null if...
// - foo() returns null,
// - or if foo() is non-null, but bar() returns null,
// - or if foo() and bar() are non-null, but baz() returns null.
// vice versa, return value is non-null if and only if foo(), bar() and baz() are non-null
foo()?.bar()?.baz()

• ?: (the null coalescing operator) is a binary operator that returns the first operand, if non-null, else the second operand. It is often referred to as the Elvis operator, due to its resemblance to an emoticon representation of Elvis Presley.

fun sayHello(maybe: String?, neverNull: Int) {
    // use of Elvis operator
    val name: String = maybe ?: "stranger"
    println("Hello $name")
}

Lambdas

Kotlin supports higher-order functions and anonymous functions, or lambdas.^[28]

// the following function takes a lambda, f, and executes f passing it the string "lambda"
// note that (String) -> Unit indicates a lambda with a String parameter and Unit return type
fun executeLambda(f: (String) -> Unit) {
    f("lambda")
}

Lambdas are declared using braces, { }. If a lambda takes parameters, they are declared within the braces and followed by the -> operator.

// the following statement defines a lambda that takes a single parameter and passes it to the println function
val l = { c : Any? -> println(c) }
// lambdas with no parameters may simply be defined using { }
val l2 = { print("no parameters") }

"Hello world" example

(Taken from and explained at https://kotlinlang.org/docs/kotlin-tour-hello-world.html.)

fun main() {
    println("Hello, world!")
    // Hello, world!
}