Test-Driven Binary Search Tree with Kotlin, Part 1

Hi, and welcome to my first post on my new blog. Today, we'll be covering test-driven development and some computer science all using the Kotlin programming language. I don't have a comment section figured out, yet, so for now, if you have feedback please contact me via Twitter.

Quick Test-Driven Development (TDD) Crash Course

Alright, for those who don't know, or maybe need a refresher, test-driven development is a process for writing better code by writing the tests first. Test-driven development follows a three-step process:

  1. Write the test. See it fail.
  2. Write or change some code to make the test pass.
  3. Once you're green, refactor the code you wrote so it isn't just a mad grab at the easiest thing you could do to make your tests pass.

This process ensures that you know your code works, at least as well as you've tested it. Starting with a failing test gives you a place to start and a clear goal in mind: "I need to fix this thing and make the test pass." Then, when make your code changes and see the test go green, you know your code actually works because the tests failed before you wrote it and passes aftewards.

For me, this whole process boosts my developer happiness, as well. If I start with a failing test, I get to start with something that is broken. Writing the code that makes the tests all go green feels good because I fixed the broken thing.

The Data Structure: Binary-Search Tree (BST)

A binary-search tree is a special kind of tree in computer science. It has four main properties we'll concern ourselves with today:

  1. Each node the in the tree may have up to two children.
  2. All of the nodes that are left children of a given node are less than that node.
  3. All of the nodes that are right children of a given node are greater than that node.
  4. While not strictly true of all BSTs, our BST will not store duplicate values.

A note before we move on to the implementation: There are a couple different ways we could implement this BST, imperative and recursive. Briefly, the difference is that the imperative approach modifies the BST in place, while the recursive approach returns a new tree when operated on. I tend to like a functional programming style, so I'll be writing a recursive BST for this article. I'll likely go into more detail about functional programming in a future article.

The Setup

As the title mentions, we'll be using Kotlin, a relatively new programming language developed by Jetbrains. We can use Gradle to set up our project. Now, I like the JVM, and Gradle, so I'll do a separate article explaining more about creating new Gradle-based projects in the future. But, that's outside the scope of this article, so, for now, here's the build.gradle file you'll need to get started with this project, in case you'd like to code along with me.

buildscript {
  ext.kotlin_version = '1.2.61'

  repositories {

  dependencies {
      classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"

plugins {
    id 'org.jetbrains.kotlin.jvm' version '1.2.61'

apply plugin: 'kotlin'
apply plugin: 'application'
apply plugin: 'jacoco'

mainclass = 'MainKt'

defaultTasks 'run'

repositories {

test {
    testLogging {
        exceptionFormat = 'full'

jacocoTestReport {
    reports {
        html.enabled = true
        html.destination file("test-results/junit/html")

dependencies {
    compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
    testCompile "org.jetbrains.kotlin:kotlin-test"
    testCompile 'junit:junit:4.11'
    testCompile "org.jetbrains.kotlin:kotlin-test-junit:$kotlin_version"

check.dependsOn jacocoTestReport
Be sure to replace the Kotlin version with the one you have. If you're using an IDE, such as IntelliJ, it should warn you if the version I have is different from its version.

Okay, so, to finish the setup, put that build.gradle file at the root of your project directory. Then, create a series of nested directories that should end up looking like this:


And that's it for Part One! Join me in part two where we'll write our first test and start insert values into our tree.