Kotlin (programming language)

Kotlin (/ˈkɒtlɪn/)[2] is a cross-platform, statically typed, general-purpose programming language with type inference. Kotlin is designed to interoperate fully with Java, and the JVM version of Kotlin's standard library depends on the Java Class Library,[3] but type inference allows its syntax to be more concise. Kotlin mainly targets the JVM, but also compiles to JavaScript (for e.g. frontend web applications using React[4]) or native code (via LLVM), e.g. for native iOS apps sharing business logic with Android apps.[5] Language development costs are borne by JetBrains, while the Kotlin Foundation protects the Kotlin trademark.[6]

Kotlin
Kotlin-logo.svg
ParadigmMulti-paradigm: object-oriented, functional, imperative, block structured, declarative, generic, reflective, concurrent
Designed byJetBrains
DeveloperJetBrains
First appeared22 July 2011; 9 years ago (2011-07-22)
Stable release
1.4.10 / 10 September 2020; 19 days ago (2020-09-10)[1]
Typing disciplineInferred, static, strong
Platform
OSCross-platform
LicenseApache License 2.0
Filename extensions.kt, .kts, .ktm
Websitekotlinlang.org Edit this at Wikidata
Influenced by

On 7 May 2019, Google announced that the Kotlin programming language is now its preferred language for Android app developers.[7] Since the release of Android Studio 3.0 in October 2017, Kotlin has been included as an alternative to the standard Java compiler. The Android Kotlin compiler targets Java 6 by default, but lets the programmer choose to target Java 8 up to 13, for optimization,[8] or more features.[9]

HistoryEdit

In July 2011, JetBrains unveiled Project Kotlin, a new language for the JVM, which had been under development for a year.[10] JetBrains lead Dmitry Jemerov said that most languages did not have the features they were looking for, with the exception of Scala. However, he cited the slow compilation time of Scala as a deficiency.[10] One of the stated goals of Kotlin is to compile as quickly as Java. In February 2012, JetBrains open sourced the project under the Apache 2 license.[11]

The name comes from Kotlin Island, near St. Petersburg. Andrey Breslav mentioned that the team decided to name it after an island just like Java was named after the Indonesian island of Java[12] (though the programming language Java was perhaps named after the coffee).[13]

JetBrains hopes that the new language will drive IntelliJ IDEA sales.[14]

Kotlin v1.0 was released on 15 February 2016.[15] This is considered to be 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.[16]

Kotlin v1.2 was released on 28 November 2017.[17] Sharing code between JVM and JavaScript platforms feature was newly added to this release (as of version 1.4 multiplatform programming is an alpha feature[18] upgraded from "experimental"). Full-stack demo has been made with the new Kotlin/JS Gradle Plugin.[19][20]

Kotlin v1.3 was released on 29 October 2018, bringing coroutines for asynchronous programming.

On 7 May 2019, Google announced that the Kotlin programming language is now its preferred language for Android app developers.[7]

Kotlin v1.4 was released in August 2020, with e.g. some slight changes to the support for Apple's platforms, i.e. to the Objective-C/Swift interop.[21]

DesignEdit

Development lead Andrey Breslav has said that Kotlin is designed to be an industrial-strength object-oriented language, and a "better language" than Java, but still be fully interoperable with Java code, allowing companies to make a gradual migration from Java to Kotlin.[22]

Semicolons are optional as a statement terminator; in most cases a newline is sufficient for the compiler to deduce that the statement has ended.[23]

Kotlin variable declarations and parameter lists have the data type come after the variable name (and with a colon separator), similar to BASIC, Pascal and TypeScript.

Variables in Kotlin can be read-only, declared with the val keyword, or mutable, declared with the var keyword.[24]

Class members are public by default, and classes themselves are final by default, meaning that creating a derived class is disabled unless the base class is declared with the open keyword.

In addition to the classes and member functions (equivalent to methods) of object-oriented programming, Kotlin also supports procedural programming with the use of functions.[25] Kotlin functions (and constructors) support default arguments, variable-length argument lists, named arguments and overloading by unique signature. Class member functions are virtual, i.e. dispatched based on the runtime type of the object they are called on.

Kotlin 1.3 adds (experimental) support for contracts[26] (inspired by Eiffel's design by contract[27] programming paradigm)

SyntaxEdit

Procedural programming styleEdit

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

Main entry pointEdit

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[28]). Perl and Unix shell style string interpolation is supported. Type inference is also supported.

 1 // Hello, World! example
 2 fun main() {
 3     val scope = "World"
 4     println("Hello, $scope!")
 5 }
 6 
 7 fun main(args: Array<String>) {
 8     for (arg in args) {
 9         println(arg)
10     }
11 }

Extension functionsEdit

Similar to C#, Kotlin allows a user to add functions to any class without the formalities of creating a derived class with new functions. Instead, Kotlin adds the concept of an extension function which allows a function to be "glued" onto the public function list of any class without being formally placed inside of the class. In other words, an extension function is a helper function that has access to all the public interface of a class which it can use to create a new function interface to a target class and this function will appear exactly like a function of the class, appearing as part of code completion inspection of class functions. For example:

1 package MyStringExtensions
2 
3 fun String.lastChar(): Char = get(length - 1)
4 
5 >>> println("Kotlin".lastChar())

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.

1 // Overloading '+' operator using an extension function
2 operator fun Point.plus(other: Point): Point {
3     return Point(x + other.x, y + other.y)
4 }
5 
6 >>> val p1 = Point(10, 20)
7 >>> val p2 = Point(30, 40)
8 >>> println(p1 + p2)
9 Point(x=40, y=60)

Unpack arguments with spread operatorEdit

Similar to Python, the spread operator asterisk (*) unpacks an array's contents as comma-separated arguments to a function:

1 fun main(args: Array<String>) { 
2     val list = listOf("args: ", *args)
3     println(list)
4 }

Destructuring declarationsEdit

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:

1 for ((key, value) in map) {
2     println("$key: $value")
3 }

Nested functionsEdit

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

 1 class User(val id: Int, val name: String, val address: String)
 2     
 3 fun saveUserToDb(user: User) {
 4     fun validate(user: User, value: String, fieldName: String) {
 5         require(value.isNotEmpty()) { "Can't save user ${user.id}: empty $fieldName" }
 6     }
 7     
 8     validate(user, user.name, "Name") 
 9     validate(user, user.address, "Address")
10     // Save user to the database 
11     ...
12 }

Classes are final by defaultEdit

In Kotlin, to derive a new class from a base class type, the base class needs to be explicitly marked as "open". This is in contrast to most object-oriented languages such as Java where classes are open by default.

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

 1 // open on the class means this class will allow derived classes
 2 open class MegaButton  {
 3 
 4     // no-open on a function means that 
 5     //    polymorphic behavior disabled if function overridden in derived class
 6     fun disable() { ... }
 7 
 8     // open on a function means that
 9     //    polymorphic behavior allowed if function is overridden in derived class
10     open fun animate() { ... }
11 }
12 
13 class GigaButton: MegaButton {
14 
15     // Explicit use of override keyword required to override a function in derived class
16     override fun animate() { println("Giga Click!") } 
17 }

Abstract classes are open by defaultEdit

Abstract classes define abstract or "Pure Virtual" placeholder function that will be defined in a derived class. Abstract classes are open by default.

 1 // No need for the open keyword here, it’s already open by default
 2 abstract class Animated {
 3 
 4     // This virtual function is already open by default as well
 5     abstract fun animate()
 6   
 7     open fun stopAnimating() { }
 8 
 9     fun animateTwice() { }
10 }

Classes are public by defaultEdit

Kotlin provides the following keywords to restrict visibility for top-level declaration, such as classes, and for class members:

   public, internal, protected, and private.

When applied to a class member:

   public (default): Visible everywhere 
   internal:         Visible in a module 
   protected:        Visible in subclasses 
   private:          Visible in a class

When applied to a top-level declaration

   public (default):  Visible everywhere
   internal:          Visible in a module
   private:           Visible in a file

Example:

1 // Class is visible only to current module
2 internal open class TalkativeButton : Focusable {
3     // method is only visible to current class 
4     private   fun yell() = println("Hey!")
5 
6     // method is visible to current class and derived classes
7     protected fun whisper() = println("Let's talk!")
8 }

Primary constructor vs. secondary constructorsEdit

Kotlin supports the specification of 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.

1 // Example of class using primary constructor syntax
2 // (Only one constructor required for this class)
3 open class PowerUser : User (
4     protected val nickname: String, 
5     final override var isSubscribed: Boolean = true) 
6     {
7          ...
8     }

However, in cases where more than one constructor is needed for a class, a more general constructor can be used called secondary constructor syntax which closely resembles the constructor syntax used in most object-oriented languages like C++, C#, and Java.

 1 // Example of class using secondary constructor syntax
 2 // (more than one constructor required for this class)
 3 class MyButton : View {
 4 
 5     // Constructor #1 
 6     constructor(ctx: Context) : super(ctx) { 
 7         // ... 
 8     } 
 9   
10     // Constructor #2
11     constructor(ctx: Context, attr: AttributeSet) : super(ctx, attr) { 
12         // ... 
13     }
14 }

Data ClassEdit

Kotlin provides Data Classes to define classes that store only properties. In Java programming, classes that store only properties are not unusual, but regular classes are used for this purpose. Kotlin has given provision to exclusively define classes that store properties alone. These data classes do not have any methods but only properties. A data class does not contain a body, unlike a regular class. The data keyword is used before the class keyword to define a data class.

1 fun main(args: Array) {
2     // create a data class object like any other class object
3     var book1 = Book("Kotlin Programming", 250)
4     println(book1)
5     // output: Book(name=Kotlin Programming, price=250)
6 }
7      
8 // data class with parameters and their optional default values
9 data class Book(val name: String = "", val price: Int = 0)

Kotlin interactive shellEdit

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

Kotlin as a scripting languageEdit

Kotlin can also be used as a scripting language. A script is a Kotlin source file (.kts) with top level executable code.

1 // list_folders.kts
2 import java.io.File
3 val folders = File(args[0]).listFiles { file -> file.isDirectory() }
4 folders?.forEach { folder -> println(folder) }

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"

Complex "hello world" exampleEdit

 1 fun main(args: Array<String>) {
 2     greet {
 3         to.place
 4     }.print()
 5 }
 6 
 7 // Inline higher-order functions
 8 inline fun greet(s: () -> String) : String = greeting andAnother s()
 9 
10 // Infix functions, extensions, type inference, nullable types, 
11 // lambda expressions, labeled this, Elvis operator (?:)
12 infix fun String.andAnother(other : Any?) = buildString() 
13 { 
14     append(this@andAnother); append(" "); append(other ?: "") 
15 }
16 
17 // Immutable types, delegated properties, lazy initialization, string templates
18 val greeting by lazy { val doubleEl: String = "ll"; "he${doubleEl}o" }
19 
20 // Sealed classes, companion objects
21 sealed class to { companion object { val place = "world"} }
22 
23 // Extensions, Unit
24 fun String.print() = println(this)

Kotlin makes a distinction 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: null-check must be performed before using the value. Kotlin provides null-safe operators to help developers:

1 fun sayHello(maybe: String?, neverNull: Int) {
2     // use of elvis operator
3     val name: String = maybe ?: "stranger"
4     println("Hello $name")
5 }

An example of the use of the safe navigation operator:

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

Kotlin provides support for higher order functions and anonymous functions or lambdas.[29]

1 // the following function takes a lambda, f, and executes f passing it the string, "lambda"
2 // note that (s: String) -> Unit indicates a lambda with a String parameter and Unit return type
3 fun executeLambda(f: (s: String) -> Unit) {
4     f("lambda")
5 }

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

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

ToolsEdit

  • IntelliJ IDEA has plug-in support for Kotlin.[30] IntelliJ IDEA 15 was the first version to bundle the Kotlin plugin in the IntelliJ Installer, and provide Kotlin support out of the box.[31]
  • JetBrains also provides a plugin for Eclipse.[32][33]
  • Integration with common Java build tools is supported including Apache Maven,[34] Apache Ant,[35] and Gradle.[36]
  • Android Studio (based on IntelliJ IDEA) has official support for Kotlin, starting from Android Studio 3.[37]
  • Emacs has a Kotlin Mode in its Melpa package repository.
  • Vim has a plugin maintained on Github.[38]

ApplicationsEdit

Kotlin is widely used for Server Side, and Android development. On Android the platform was stuck on Java 7 for a while (with some contemporary language features made accessible through the use of Retrolambda[39] or the Jack toolchain[40]) and Kotlin introduces many improvements for programmers such as null-pointer safety, extension functions and infix notation. Accompanied by full Java compatibility and good IDE support (Android Studio[41]) it is intended to improve code readability, give an easier way to extend Android SDK classes and speed up development.[42]

Kotlin was announced as an official Android development language at Google I/O 2017. It became the third language fully supported for Android, in addition to Java and C++.[43]

Spring officially added Kotlin support[44] with version 5 on 04 January 2017 before the Android announcement made by Google at Google I/O 2017. Some of the official Kotlin support in Spring covers documentation[45], library support[46], and video presentations[47]. Many of the other Server Side libraries/frameworks like Ktor[48], and Vert.x[49] provide official Kotlin support. Google Cloud Platform officially supports Kotlin[50], however the other cloud platforms like AWS (Amazon Web Services), and Azure don't officially provide Kotlin support although Kotlin is known to run on these platforms.

Kotlin is mainly used in new projects[51] (96%), and is mostly used in work projects (75%), followed by personal projects (68%). The following types of software are developed using Kotlin:

  • Mobile (56%)
  • Web Backend (47%)
  • Library/framework (28%)
  • Tooling (20%)

Linux is the top target[52] used in Kotlin Native projects (74%), followed by Android (48%), and MacOS (47%).

AdoptionEdit

In 2018, Kotlin was the fastest growing language on GitHub with 2.6 times more developers compared to 2017.[53] It's the fourth most loved programming language according to the 2020 Stack Overflow Developer Survey.[54]

Kotlin was also awarded the O'Reilly Open Source Software Conference Breakout Award for 2019.[55]

Many companies/organisations have used Kotlin for backend development:


Some companies/organisations have used Kotlin for web development:


A number of companies have publicly stated using Kotlin:

See alsoEdit

ReferencesEdit

  • This article contains quotations from Kotlin tutorials which are released under an Apache 2.0 license.
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External linksEdit