Java interoperability

Kalm generates Java source and ordinary JVM classes. Java sees records, enums, interfaces, methods, and constructors.

Call Java

use java.time.LocalDate

pub fun year() -> Int = LocalDate.now().getYear()

Java visibility, overloads, varargs, boxing, and SAM lambdas retain their usual invocation rules. Public Kalm types belong in matching .kalm files.

Call Kalm

Top-level functions in Greeting.kalm become methods of GreetingKalm. Both source sets can call each other within the same build.

pub fun greeting(name: Text) -> Text = @"Hello, $name!"

String message = GreetingKalm.greeting("Mira");

Nullability

Unannotated Java references are platform values. Give a potentially absent result an explicit nullable type before coalescing it. Assigning an unannotated reference to a non-null target produces a warning.

pub fun projectName() -> Text:
    fix configured: Text? = System.getProperty("project.name")
    return configured ?? "untitled"

Generated signatures preserve nullability metadata, including generic arguments and array dimensions, for later Kalm compilations.

Resources and errors

Handle checked Java exceptions with try/handle. Use with for closeable resources. Resources and concurrency explains both.

An external fun wraps a Java static method under a Kalm name. An ordinary augment adds Kalm extension functions; bridge augment also exposes a public helper facade to Java.

Build integration

Use Maven or Gradle to compile Java and Kalm source sets together. See implementation limitations for boundaries that affect existing Java projects.