What is Kotlin? The Java alternative

 Kotlin is a general purpose, free, open source, statically typed “pragmatic” programming language initially designed for the JVM (Java Virtual Machine) and Android that combines object-oriented and functional programming features. It is focused on interoperability, safety, clarity, and tooling support. Versions of Kotlin targeting JavaScript ES5.1 and native code (using LLVM) for a number of processors are in production as well.

Kotlin originated at JetBrains, the company behind IntelliJ IDEA, in 2010, and has been open source since 2012. The Kotlin team currently has more than 90 full-time members from JetBrains, and the Kotlin project on GitHub has more than 300 contributors. JetBrains uses Kotlin in many of its products including its flagship IntelliJ IDEA.

Kotlin as a more concise Java language

At first glance, Kotlin looks like a more concise and streamlined version of Java. Consider the screenshot above, where I have converted a Java code sample (at left) to Kotlin automatically. Notice that the mindless repetition inherent in instantiating Java variables has gone away. The Java idiom

StringBuilder sb = new StringBuilder();

Becomes in Kotlin

val sb = StringBuilder()

You can see that functions are defined with the fun keyword, and that semicolons are now optional when newlines are present. The val keyword declares a read-only property or local variable. Similarly, the var keyword declares a mutable property or local variable.

Nevertheless, Kotlin is strongly typed. The val and var keywords can be used only when the type can be inferred. Otherwise you need to declare the type. Type inference seems to be improving with each release of Kotlin.

Have a look at the function declaration near the top of both panes. The return type in Java precedes the prototype, but in Kotlin it succeeds the prototype, demarcated with a colon as in Pascal.

It is not completely obvious from this example, but Kotlin has relaxed Java’s requirement that functions be class members. In Kotlin, functions may be declared at top level in a file, locally inside other functions, as a member function inside a class or object, and as an extension function. Extension functions provide the C#-like ability to extend a class with new functionality without having to inherit from the class or use any type of design pattern such as Decorator.

For Groovy fans, Kotlin implements builders; in fact, Kotlin builders can be type checked. Kotlin supports delegated properties, which can be used to implement lazy properties, observable properties, vetoable properties, and mapped properties.

Many asynchronous mechanisms available in other languages can be implemented as libraries using Kotlin coroutines. This includes async/await from C# and ECMAScript, channels and select from Go, and generators/yield from C# and Python.

Kotlin for Android

Up until May 2017, the only officially supported programming languages for Android were Java and C++. Google announced official support for Kotlin on Android at Google I/O 2017, and starting with Android Studio 3.0 Kotlin is built into the Android development toolset. Kotlin can be added to earlier versions of Android Studio with a plug-in.

Kotlin compiles to the same byte code as Java, interoperates with Java classes in natural ways, and shares its tooling with Java. Because there is no overhead for calling back and forth between Kotlin and Java, adding Kotlin incrementally to an Android app currently in Java makes perfect sense. The few cases where the interoperability between Kotlin and Java code lacks grace, such as Java set-only properties, are rarely encountered and easily fixed.

Pinterest was the poster child for Android apps written in Kotlin as early as November 2016, and it was mentioned prominently at Google I/O 2017 as part of the Kotlin announcement. In addition, the Kotlin team likes to cite the Evernote, Trello, Square, and Coursera apps for Android.