1. From Kotlin code to the JVM · The JVM and the Kotlin compiler

[THEORY]

[PREDICT]

Read the code carefully before you run it.

prediction.kt

// File: Utils.kt
package de.firma.tools

fun square(x: Int): Int = x * x

class Calculator {
    fun double(x: Int): Int = x * 2
}

How many .class files does the Kotlin compiler produce from this one Utils.kt, and what are they called?

[TASK]

AppConfig.kt contains only top-level declarations. Java code currently has to call them through AppConfigKt.loadConfiguration() — after all, the generated class is named after the file plus "Kt". Use a file annotation to make the generated class be called AppConfig instead, without renaming the file or moving the code into a real class.

task.kt

// TODO: add the annotation so the generated class is named "AppConfig" instead of "AppConfigKt" package de.firma.konfiguration const val MAX_ATTEMPTS = 3 fun loadConfiguration(): String = "Standard configuration"

Before you get a hint: what have you tried, and where exactly are you stuck?

[COMPREHENSION CHECK]

1. Why can Java code call Kotlin top-level functions at all, although the JVM knows no "free" functions outside classes?

The compiler wraps each top-level function in its own singleton object that Java code reaches through an INSTANCE field. The JVM attaches functions outside a class to java.lang.Object, so Java code can call them on any instance. The compiler puts a file's top-level functions into a synthetic class, named after the file plus Kt, as static methods. The Kotlin runtime registers all top-level functions when the JVM starts, and Java code finds them by name through reflection.

2. What does the JIT compiler (Just-in-Time) do when the JVM recognizes a method as "hot" (executed often)?

It caches the return values of the method, so repeated calls with the same arguments skip the execution. It switches the whole JVM from interpreting to compiling, so every other method runs as machine code from then on. It translates the method's bytecode into native machine code while the program runs, so later calls are faster. It recompiles the Kotlin source of the method with extra optimizations and overwrites the .class file on disk.

3. A file Utils.kt contains a top-level function square() AND a class Calculator. How many .class files does the Kotlin compiler produce from that, and what are they called?

Two: Utils.class for the contents of the file and Calculator.class for the class. One: UtilsKt.class, which holds both the function square() and the class Calculator. Three: Utils.class for the file, UtilsKt.class for the function and Calculator.class for the class. Two: UtilsKt.class for the top-level function and Calculator.class for the class.