Kotlin (programming language)

Kotlin

Designed by	JetBrains
Developer	JetBrains and open source contributors
Stable release	Kotlin 1.0.1 / March 16, 2016 (2016-03-16)
Preview release	Kotlin 1.0.2 EAP 44 / April 16, 2016 (2016-04-16)
Typing discipline	static
Platform	Outputs Java Virtual Machine bytecode and JavaScript source code
OS	any supporting a JVM or JavaScript interpreter
License	Apache 2
Filename extensions	.kt, .kts
Website	kotlinlang.org
Influenced by
Java, Scala, Groovy, C#, Gosu

Kotlin is a statically-typed programming language that runs on the Java Virtual Machine and also can be compiled to JavaScript source code. Its primary development is from a team of JetBrains programmers based in Saint Petersburg, Russia (the name comes from the Kotlin Island, near St. Petersburg). Kotlin was named Language of the Month in the January 2012 issue of Dr. Dobb's Journal. While not syntax compatible with Java, Kotlin is designed to interoperate with Java code and is reliant on Java code from the existing Java Class Library, such as the collections framework.

History

In July 2011 JetBrains unveiled Project Kotlin, a new language for the JVM, which had been under development for a year. 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 compile time of Scala as an obvious deficiency. 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. Jetbrains hopes that the new language will drive IntelliJ IDEA sales.

Kotlin v1.0 was released on February 15, 2016. This is considered to be the first officially stable release and JetBrains has committed to long-term backwards compatibility starting with this version.

Philosophy

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

Syntax

Like Pascal, TypeScript, Haxe, PL/SQL, F#, Go and Scala—and unlike C and its derivatives such as C++, Java, C#, and D—Kotlin variable declarations and parameter lists have the data type come after the variable name (and with a colon separator). As in other modern languages such as Scala and Groovy, semicolons are optional as a statement terminator; in most cases a newline is sufficient for the compiler to deduce that the statement has ended.

Semantics

In addition to the classes and methods (called member functions in Kotlin) of object-oriented programming, Kotlin also supports procedural programming with the use of functions. As in C and C++, the entry point to a Kotlin program is a function named "main", which is passed an array containing any command line arguments. Perl and Unix/Linux shell script-style string interpolation is supported. Type inference is also supported.

Hello, world! example

fun main(args : Array<String>) {
  val scope = "world"
  println("Hello, $scope!")
}

Kotlin makes a distinction between nullable and non-nullable datatypes. 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:

• ?. (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.
• ?: (null coalescing operator) often referred to as the Elvis operator:

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

An example of the use of the safe navigation operator:

  // returns null if foo is null, or bar() returns null, or baz() returns null
  foo ?. bar() ?. baz()

Tools

• IntelliJ IDEA has plug-in support for Kotlin. IntelliJ IDEA 15 is the first version to bundle Kotlin plugin in the IntelliJ Installer, and provide support of it out of the box.
• JetBrains also has an Eclipse plug-in.
• Apache Maven plugin.
• Apache Ant task.
• Gradle plugin.

See also

There are other languages intended to be a better language than Java for the JVM:

• Ceylon (programming language)
• Clojure
• Fantom (programming language)
• Gosu (programming language)
• Scala (programming language)
• Groovy (programming language)
• Xtend

Related concepts:

• Nullable type
• Operator overloading

Applications

One of the obvious applications of Kotlin is Android development. The platform has been long stuck on Java 6 (note: retrolambda and the transition to Jack compiler) and Kotlin introduces many improvements for programmers such as null-pointer safety, extension functions and infix notation. Accompanied by 100% Java compatibility and good IDE support (Android Studio) it gives an excellent way to improve code readability, extend Android SDK classes and speed up development.

Users

According to the Kotlin website, Prezi is using Kotlin in the backend.

External links

• Web-Demo and examples

• https://discuss.kotlinlang.org/t/kotlin-1-0-2-eap/1581/2
• https://www.jetbrains.com/idea/whatsnew/
• Kotlin and the search for a better Java, Stephen Colebourne, 2011-07-21.