Skip to main content

Calling Kotlin from Java

File functions​

// File: Formatters.kt
fun formatName(name: String): String = name.trim()

Java normally calls FormattersKt.formatName(name). @file:JvmName("Formatters") changes the generated class name.

Defaults and overloads​

Java cannot omit Kotlin default arguments directly.

@JvmOverloads
fun greet(name: String, prefix: String = "Hello") = "$prefix, $name"

@JvmOverloads generates Java overloads by removing trailing default parameters. Use it only where Java ergonomics justify the expanded API.

Static-looking members​

class Factory {
    companion object {
        @JvmStatic fun create(): Factory = Factory()
    }
}

Java may call Factory.create(). Without @JvmStatic, it calls through Factory.Companion.

Fields and names​

  • @JvmField: expose a property as a field when appropriate.
  • @JvmName: change generated JVM method name, often to resolve signature clashes.
  • @Throws(IOException::class): declare checked exceptions for Java callers.

Function types from Java​

Kotlin function types appear as FunctionN interfaces to Java and can be awkward. For public Java-facing callbacks, a named fun interface may be clearer.

fun interface UserCallback { fun onUser(user: User) }

✕ Design only for Kotlin, then expose accidentally

fun load(callback: (Result<User>) -> Unit)

✓ Review the generated Java API

fun interface LoadCallback { fun onResult(result: UserResult) }
fun load(callback: LoadCallback)
Interview-ready answer

Kotlin is callable from Java, but defaults, companions, file functions, function types, checked exceptions, and name mangling affect ergonomics. Use JVM annotations deliberately and test the API from Java source.