Java/Kotlin Interop

JVM compatibility

Skotch produces standard Java 17 class files (major version 61). Compiled .class files run on any JVM that supports Java 17 or later.

Top-level Kotlin functions are compiled to static methods on a wrapper class named after the source file (e.g., main.kt → MainKt). This matches the convention used by JetBrains’ kotlinc.

skotch emit --target jvm hello.kt -o HelloKt.class
java -cp . HelloKt

DEX compatibility

Skotch produces DEX v035 bytecode, compatible with Android API 13+. The output can be included in an APK and run on Android devices or emulators.

Calling Java static methods

Skotch supports calling static methods on well-known Java classes directly:

fun main() {
    val timestamp = System.currentTimeMillis()
    println(timestamp)

    println(Math.abs(-42))        // 42
    println(Math.max(10, 20))     // 20
    println(Integer.parseInt("123")) // 123
}

This also works in the REPL:

skotch> println(System.currentTimeMillis())
1744385123456

Fully-qualified names

You can also use fully-qualified Java class names:

fun main() {
    println(java.lang.System.getProperty("file.separator"))
    println(java.lang.Math.random())
}

Import statements

Explicit imports are supported:

import java.lang.Math
import java.lang.Integer

fun main() {
    println(Math.abs(-42))
    println(Integer.parseInt("123"))
}

java.lang.* is implicitly imported, matching Kotlin’s default imports.

Supported Java classes

Class	Methods	Notes
System	currentTimeMillis(), nanoTime(), exit(Int), getProperty(String), getenv(String)	java.lang.System
Math	abs(Int), max(Int,Int), min(Int,Int), random(), sqrt(Double), pow(Double,Double)	java.lang.Math
Integer	parseInt(String), toString(Int), valueOf(Int)	java.lang.Integer
Long	parseLong(String)	java.lang.Long
String	valueOf(Int)	java.lang.String
Thread	sleep(Long)	java.lang.Thread

Tip

Method signatures are resolved from real .class files at compile time. The compiler reads from:

1. JDK jmod files — all *.jmod in the JDK’s jmods/ directory
2. CLASSPATH — directories and JAR files on the CLASSPATH env var
3. Kotlin stdlib — kotlin-stdlib.jar located via KOTLIN_HOME, CLASSPATH, or by resolving the kotlinc binary on PATH

Instance methods on built-in types (String.length, String.substring(), etc.) are resolved dynamically from java.lang.String’s class file. Kotlin-specific extension functions (like String.uppercase()) are mapped to their JVM equivalents (String.toUpperCase()) via a small translation table.

Current limitations

Feature	Status
Calling arbitrary Java libraries	JDK classes resolved from jmod files; CLASSPATH JARs/dirs scanned
import package.Class	Working — resolves class names for static calls
import package.*	Parsed but classes not enumerated (hardcoded set only)
External dependency resolution	Not yet — Maven/Gradle deps ignored
Kotlin standard library	Not bundled; println is a compiler intrinsic
Reflection	Not supported
Annotations	Not yet compiled

What works today

• Compiled .class files are valid Java bytecode and can be loaded by standard JVM tools (java, javap, jstack, etc.)
• The REPL executes compiled code in an embedded JVM via JNI
• Multiple Skotch-compiled files can call each other’s functions via invokestatic
• Static method calls on System, Math, Integer, and String via invokestatic