Kotlin

Kotlin is a statically typed, multi-paradigm language developed by JetBrains and released in 2016. It compiles to JVM bytecode, JavaScript or native code, and it interoperates fully with existing Java libraries.

Why it matters

Google made Kotlin its preferred language for Android development in 2019, and it is also used for server-side work with frameworks such as Spring and Ktor. Compared with Java it is more concise: data classes, type inference, default arguments and extension functions remove boilerplate. Its type system separates nullable from non-nullable types, which eliminates many NullPointerException errors at compile time.

Example

data class Site(val name: String, val up: Boolean)

fun main() {
    val sites = listOf(Site("a", true), Site("b", false))
    val down = sites.filter { !it.up }.map { it.name }
    println(down)   // [b]
}

The data class generates equals, toString and copy automatically, and it names the lambda parameter.

Common mistakes

• Using !! to force non-null values, which throws if the value is null.
• Calling Java APIs without realizing their return types may be null ("platform types").
• Blocking the main thread in Android instead of using coroutines.

Related

• Java
• Null
• Android
• Async programming

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