In most cases, calling Java code from Kotlin works the way you might expect. For example, any time both getters and setters exist in a Java class, Kotlin treats them as properties with the same name. Similarly, Boolean accessor methods are treated as properties that have the same name as the getter method. For example:
import java.util.Calendar
fun calendarDemo() {
val calendar = Calendar.getInstance()
if (calendar.firstDayOfWeek == Calendar.SUNDAY) { // call getFirstDayOfWeek()
calendar.firstDayOfWeek = Calendar.MONDAY // call setFirstDayOfWeek()
}
if (!calendar.isLenient) { // call isLenient()
calendar.isLenient = true // call setLenient()
}
}
This scheme breaks down for the case of Java set-only properties, which are not yet supported in Kotlin. If a Java class has only a setter, it will not be visible as a property in Kotlin.
Kotlin’s interoperability with Java extends to Java tools. Kotlin doesn’t have its own editors or IDEs; it has plug-ins for the popular Java editors and IDEs, including IntelliJ IDEA, Android Studio, and Eclipse. Kotlin doesn’t have its own build system; it uses Gradle, Maven, and Ant.
Kotlin interoperability with JavaScript
You can use Kotlin to write code for browsers and Node.js. With this target, Kotlin is transpiled to JavaScript ES5.1 instead of being compiled to JVM byte code.
Because JavaScript is a dynamic language, Kotlin for JavaScript adds a dynamic type not available in Kotlin for the JVM:
val dyn: dynamic = ...
The Kotlin type checker ignores dynamic types and lets JavaScript deal with them at runtime. Expressions using values of dynamic type are translated to JavaScript as written. Again, Kotlin basically punts and lets JavaScript interpret the expressions at runtime.
You can call JavaScript from Kotlin two ways: using dynamic types or using strongly typed Kotlin headers for JavaScript libraries. To access well-known, third-party JavaScript frameworks with a strongly typed API, you can convert TypeScript definitions from the DefinitelyTyped type definitions repository to Kotlin using the ts2kt tool.
You can inline JavaScript code as text into your Kotlin code using the js("…") function. You can mark package declarations in Kotlin code as pure JavaScript with the external modifier. You can call Kotlin code from JavaScript, but you need to use fully qualified names.
Kotlin applications
Kotlin can be used for any kind of development: multi-platform, server-side, client-side web, Android, native code, data science, and even competitive programming. According to the Kotlin Census 2020, the latest Kotlin developer survey conducted by JetBrains, the most popular uses for Kotlin are mobile development (63%), web back-end development (40%), library or framework development (17%), tooling (9%), desktop (5%), web front end (5%), and systems programming (5%). The most popular platform targets are Kotlin/Android (65%), Kotlin/JVM server (48%), Kotlin/JVM other (15%), Kotlin/Native (10%), and Kotlin/JS (6%).
Overall, Kotlin offers several advantages over Java for code to be run on the JVM, and Kotlin can also be used to generate JavaScript and native code. Compared to Java, Kotlin is safer, more concise, and more explicit. It supports functional programming in addition to object-oriented programming, offers a bunch of useful features (extension functions, builders, coroutines, lambdas, etc.), and provides null safety through nullable and non-nullable types. Best of all, if you already know Java, you’ll be productive in Kotlin in no time.