Kotlin Koans—Part 2

After doing the first tutorial on Kotlin, I was impressed, but let’s face it, anyone can do a simple “hello world” style program. Nevertheless, I decided to continue with the Kotlin tutorial found at kotlinlang.org. When I moved onto part two of the tutorial, I was really impressed.

It wasn’t that I was super impressed with the language itself. It was IntelliJ’s support of Kotlin that blew me away. When you copy and paste Java code into the IDE, it will offer to translate it to Kotlin for you.

You can see in the video that IntelliJ just did the work of taking the Java code that I copied and pasted into my Kotlin class. I thought this was incredibly slick because it gave me the change to see the differences between Java and Kotlin.

Of course, I wanted to do the exercise myself so that I can get the hang of writing Kotlin code. The problem in this portion of the tutorial was to take this Java code and rewrite as Kotlin code.

public class JavaCode1 extends JavaCode {
    public String task1(Collection collection) {
        StringBuilder sb = new StringBuilder();
        Iterator iterator = collection.iterator();
        while (iterator.hasNext()) {
            Integer element = iterator.next();
            if (iterator.hasNext()) {
                sb.append(", ");
        return sb.toString();

It’s not too painful of code. Here is what I ended with when I wrote the code as Kotlin code on my own.

fun todoTask1(collection: Collection): Nothing = TODO(
        Task 1.
        Rewrite JavaCode1.task1 in Kotlin.
        In IntelliJ IDEA, you can just copy-paste the code and agree to automatically convert it to Kotlin,
        but only for this task!
    references = { JavaCode1().task1(collection) })

fun task1(collection: Collection): String {
    val sb = StringBuilder()
    val iterator = collection.iterator()
    while (iterator.hasNext()){
        val element = iterator.next()
        if (iterator.hasNext()){
            sb.append(", ")
    return sb.toString()

There was one thing I noticed about the Kotlin code that I liked. It looks as if we are allowed to have free standing functions in Kotlin outside of a class definition. While I appreciate OOP, there are frankly times where I’m not sure if OOP is the best approach to a problem. This was one of things I really like about Python is that I can break out of OOP when I want.

Now I know that it’s perfectly true that we can use static imports in Java, but I have always felt that was a clumsy approach. Static functions and static imports always seemed more like an after thought to the language that got tacked on after enough people complained. Of course, that’s just a matter of opinion, but anyway, I do like having a choice in Kotlin about when to use classes or just when to use function. Kotlin seems to have included this choice as part of the design of the language right from the get go.

You can click here to see Part 1 and here to see Part 3.


Kotlin Koans—Part 1

I read that Android is going to officially support Kotlin now. Last year, I bought the IntelliJ IDE and one of the first things I noticed was that the IDE offered to make Kotlin classes. I had never done anything with Kotlin but I often wondered about it. It looked interesting to me, but now that Google has thrown in with Kotlin, I decided to give it try for myself.

This is the first in a series of posts where I’m going to work through the tutorials provided on kotlinlang.org. This first post was on the very first tutorial, which is the classical ‘Hello World’ style problem.

I started by cloning the github project that they give you. Here is what I got presented with.

package i_introduction._0_Hello_World

import util.TODO
import util.doc0

fun todoTask0(): Nothing = TODO(

        Kotlin Koans project consists of 42 small tasks for you to solve.
        Typically you'll have to replace the function invocation 'todoTaskN()', which throws an exception,
        with the correct code according to the problem.

        Using 'documentation =' below the task description you can open the related part of the online documentation.
            Press 'Ctrl+Q'(Windows) or 'F1'(Mac OS) on 'doc0()' to call the "Quick Documentation" action;
            "See also" section gives you a link.
            You can see the shortcut for the "Quick Documentation" action used in your IntelliJ IDEA
            by choosing "Help -> Find Action..." (in the top menu), and typing the action name ("Quick Documentation").
            The shortcut in use will be written next to the action name.

        Using 'references =' you can navigate to the code mentioned in the task description.

        Let's start! Make the function 'task0' return "OK".
    documentation = doc0(),
    references = { task0(); "OK" }

fun task0(): String {
    return todoTask0()

My job was to make the function task0 was to make it return “OK”. It wasn’t too painful. I just had to update task0

fun task0(): String {
    return "OK"

Once I did this, I ran the unit test that they give you to check if you did the task properly. This was easy to do in IntelliJ. The IDE provides you with a button to click on to run the test.
After I ran the test, I got the output the test was expecting.