kotlin - a new hope

I’ve been looking for Java alternatives since my first days of Android development. I can’t say I don’t like Java. It’s a nice language, very easy to learn and pretty logical. But the lack of proper functional style constructions often inspires me to check what’s new in the field of JVM languages.

So far I’ve tried (tried, I never learnt these languages properly):

This week I discovered Kotlin, a language made by JetBrains (guys making IntelliJ IDEA). The history of Kotlin starts in 2010, but Android support was added in 2012. The ideas behind this language are to make it safer than Java and faster/easier than Scala.

As a demo app I was going to make a theremin-like app that played different notes when phone was tilted.

Kotlin - The Phantom Menace

I’ve heard Kotlin has nice support of the IDE, but as a convinced vim user I decided to try it in console/vim environment.

So, on the official website I found no binaries. Instead, I found a link to Github release with source archives. That was not looking promising, as I have no installed Maven, Gradle or whatever is used these days to make Java world more complicated. Luckily, Kotlin build system was plain old Ant.

Dealing with dependencies and compiling the compiler took a while, and in the $SRC/dist/kotlinc directory I found compiler binaries and runtime. Also, in the $SRC/dist/kotlinc/tools directory there was a j2k app (I thought it’s a translator from Java to Kotlin), but it failed to run.

Kotlin - Empire Strikes Back

So, let’s get back to Google stuff and make kotlin compiler work with Android build.xml. Customizations of build process are possible with custom_rules.xml. Again, good design of kotlin provides us with kotlinc Ant task, so we need to include that task into the custom_rules.xml.

So, starting with android create project as usual, I added the following custom rules:

<?xml version="1.0" encoding="UTF-8"?>
<project name="scala-android">
    <property environment="env"/>
    <taskdef resource    = "org/jetbrains/jet/buildtools/ant/antlib.xml">
            <fileset dir = "${env.KOTLIN_HOME}/lib" includes = "*.jar"/>
    <target name="-post-compile">
        <path id="project.javac.classpath">
            <path refid="project.all.jars.path" />
            <path refid="tested.project.classpath" />
            <path path="${java.compiler.classpath}" />
            <path path="${project.target.android.jar}"/>
            <path path="${out.classes.absolute.dir}" />
        <kotlinc src="src" output="bin/classes" 
            classpathref="project.javac.classpath" />

I’m not good at Ant, but that works if KOTLIN_HOME environment variable is set correctly (it should point to the kotlinc folder). We use post-install hook, because R.java should be generated and compiled before compiling kotlin files.

Then, copy Kotlin runtime from $KOTLIN_HOME/libs to your project libs directory. Runtime is small, so I hoped apk size won’t get large, too.

Now, create an empty source file src/<...package...>/Main.kt.

Kotlin - Return of the Jedi

So, let’s write some Kotlin. Minimal activity example looks much like Java:

package kotlin.theremin

import android.os.Bundle
import android.app.Activity

class Main(): Activity() {
    protected override fun onCreate(savedInstanceState : Bundle?) {

Semicolons are not needed anymore, methods are functions inside classes, inheritance looks a bit different as well as super calls.

Interesting part here is savedInstanceState : Bundle?. Question mark means a null-safe type. Also, parameter name is not recommended to be changed, since Kotlin may use named arguments in calls. That’s very annoying if careless Android SDK developers defined arguments as “p0”, “p1” and so on. Good news it’s only a warning, so I decided to ignore it when default parameter name is meaningless.

Build, install, run - all should work. Build process doesn’t feel slow, but compiler output (warnings, errors) is a bit hard to read, since at the end of the build logs there is a long stack trace if your code has errors, so I had to scroll up to see actual error messages.

Kotlin - Attack of the Clones

So, I could write some code in Kotlin. The strongest feeling was that Kotlin is a copy of Scala. I can’t say it’s bad. I think I would have spent more time to study Scala if it was not that slow, and if I saw real profit of how Scala can help me in Android development. Here’s the code of theremin app in kotlin:

package kotlin.theremin

import android.content.pm.ActivityInfo
import android.util.Log
import android.hardware.SensorEventListener
import android.hardware.SensorEvent
import android.hardware.SensorManager
import android.content.Context
import android.app.Activity
import android.os.Bundle
import android.media.AudioFormat
import android.media.AudioManager
import android.media.AudioTrack
import android.hardware.Sensor
import android.hardware.SensorEvent

class Main(): Activity() {

    val tag = "Theremin:Main"

    val audioThread = AudioThread()
    var sensorManager : SensorManager? = null
    var sensor : Sensor? = null

    val sensorListener = object : SensorEventListener {
        override fun onSensorChanged(e : SensorEvent?) {
            val values = e?.values
            if (values != null) {
                val range = sensor!!.getMaximumRange()
                var x = values[1]
                if (x < -90 || x > 90) {
                x = (x + 90) / 180
                Log.d(tag, "x = ${x}")
        override fun onAccuracyChanged(sensor : Sensor?, accuracy: Int) {}

    protected override fun onCreate(savedInstanceState : Bundle?) {


        sensorManager = getSystemService(Context.SENSOR_SERVICE) as SensorManager?

        sensor = sensorManager?.getDefaultSensor(Sensor.TYPE_ORIENTATION)
        sensorManager?.registerListener(sensorListener, sensor,


    protected override fun onDestroy() {

class AudioThread(): Thread() {

    val sampleRate = 44100

    var pitch : Int = 440

    public override fun run() {
        val buffsize = AudioTrack.getMinBufferSize(sampleRate, 
                AudioFormat.CHANNEL_OUT_MONO, AudioFormat.ENCODING_PCM_16BIT)/32
        val audioTrack = AudioTrack(AudioManager.STREAM_MUSIC,
                                                sampleRate, AudioFormat.CHANNEL_OUT_MONO,
                                                AudioFormat.ENCODING_PCM_16BIT, buffsize,
        val samples = ShortArray(buffsize)
        val amp = 10000
        val twopi = 8*Math.atan(1.toDouble())
        var ph = 0.0

        while (!Thread.interrupted()) {
            var frequency = pitch
            for (i in 0..buffsize-1) {
                samples[i] = (amp*Math.sin(ph)).toShort()
                ph += twopi*frequency/sampleRate
            audioTrack.write(samples, 0, buffsize)

    public fun setPitch(p : Float) {
        val note = Math.round(p * 8).toInt()
        val scale = array(0, 2, 4, 5, 7, 9, 11, 12)
        pitch = (Math.pow(2.0, scale[note].toDouble()/12) * 440).toInt()

It was all in one source file, Main.kt. It plays major scale notes as you tilt the phone. Sine generator code was stolen somewhere from the web, so it’s not an example of how one should write it. But I hope it’s enough to get first impression of how Kotlin looks like.

Good news: overhead is only 200KB, Kotlin runtime is very small.

Kotlin - may the force be with you

Impressions. Very nice language. Logical, consistent, fast, small. Pretty stable. I think I will spend some time learning it.

What I didn’t like: null-safety is rather painful, than safe. Most android objects coming from the outside are marked as null-safe, so I had to write “?” everywhere or check for null. However, the risk of getting null in those cases is almost zero, so I would prefer not to care about it. But language prevents such flippancy.

Converting types in math is also verbose, and often doesn’t work automatically (well, not in the way I assumed it to work). I had to split expression into smaller ones to make them compile.

Arrays are different. There is IntArray (like Java array) with no proper documentation on how to work with it, there is Array<Int>, which is Kotlin array, and they are different.

I haven’t found a way how to declare a volatile variable.

However, theoretical possibility to make domain-specific functions and extreme flexibility of the language is really amazing.

So, don’t be afraid to try Kotlin, the more developers know about it, the larger is the community - the better is the language and tools. Hope you will like it.

P.S. I will upload project sources and apk on bitbucket soon.

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Nov 28, 2013

See also: AAML - another android markup language and more.