Kotlin Koans—Part 11

This portion of the Kotlin Koans tutorial focuses on Object Expressions. Practically speaking, Object Expressions serve the same role as anonymous innner classes in Java. They let us make modifications on a class in one particular case without having to create an entirely new class.

This portion of the tutorial has developers creating a dynamic Comparator class that sorts numbers in descending order.

fun task10(): List {
    val arrayList = arrayListOf(1, 5, 2)
    Collections.sort(arrayList, object: Comparator {
        override fun compare(o1: Int?, o2: Int?): Int {
            return o2?.compareTo(o1 ?: 0) ?: 0
    return arrayList

We could have used a lambda in this case, but that would miss the point of what the tutorial is trying to teach. In this code snippet, the second paramter of Collections.sort is an Object Expression that defines a custom Comparator class.

You’ll notice that the definition of compare is full of null safe expressions as indicated the by ? and ?: opeartors. As a side note, I really like how Kotlin has an arrayListOf() function that let’s you create an ArrayList. Sure it does the same thing as Arrays.asList, but again, it’s more concise.

You can view part 10 here


Kotlin Koans—Part 10

This part of the Kotlin Koans tutorial involved extension functions. This is a construct I have never seen in programming before, so it took me a little bit to get an idea of what it is and when to use this feature.

It seems as if the idea here is to add features to a class without have to use inheritence or some sort of delegate object. Here is the Kotlin code.

//This is the class we are adding to
data class RationalNumber(val numerator: Int, val denominator: Int)

//We are adding an r() method to Int which
//returns an instance of RationalNumber
fun Int.r(): RationalNumber = RationalNumber(toInt(), 1)

//We add an r() method to Pair which returns an
//instance of RationalNumber
fun Pair.r(): RationalNumber = RationalNumber(first, second)

The Kotlin documentation has a motivation section that explains the purpose behind extensions. They explain that in many cases in Java, we end up with FileUtils, StringUtils, *Utils classes. In the ideal world, we would want to add features to say the List class directly rather than having a ListUtils class with a bunch of static methods.

We get something like this in JDK8 with default methods that can get placed in an interface. However, that still requires us to extend and interface to add extra methods. Extensions let us work directly on the classes we are already using.

You can click here to see Part 9

Kotlin Koans—Part 9

Java and Kotlin are strongly typed languages. It’s not necessary to cast types when working up an object graph. For example

public void sort(Collection col){

sort(new ArrayList());
sort(new HashSet());

This is an example of polymorphism in Java. ArrayList and HashSet are both Collections so it’s acceptable to pass either types to the example sort method.

Keep in mind this is not a two way street. This code would not compile.

public void sort(List list){

Collection col = new ArrayList();
sort(col); //Compile error!
sort((List) col); //OK

Even though col points at an ArrayList and ArrayList implements List, Java forbids you to pass col to sort without a cast. This is because the compiler has no idea that col is pointing at an ArrayList. Keep in mind this is true of Kotlin also.

Although we can get our code to compile with a cast, it’s still dangerous code. Let’s tweak it a little big and have col point at a HashSet instead of ArrayList.

public void sort(List list){

Collection col = new HashSet();

//Compiles but throws
sort((List) col);

Now the code compiles, but it will fail at run time. There is no way to cast HashSet to a List. HashSet does not implement List in anyway so when the code attempts to make the cast, the code will fail. We have to use the instanceof operator to make sure the cast is safe first.

public void sort(List list){

Collection col = new HashSet();

if (col instanceof List){
    //Now it's safe
    sort((List) col);

This code is now safe. It will check if the runtime type of col is a List first. If the object is a List, it will make the cast. Otherwise, the cast will not get made.


This portion of the Kotlin Koans tutorial shows off how Kotlin handles casting compared to Java. Here is the Java code that needs to get rewrote in Kotlin.

public class JavaCode8 extends JavaCode {
    public int eval(Expr expr) {
        if (expr instanceof Num) {
            return ((Num) expr).getValue();
        if (expr instanceof Sum) {
            Sum sum = (Sum) expr;
            return eval(sum.getLeft()) + eval(sum.getRight());
        throw new IllegalArgumentException("Unknown expression");

Kotlin has a when keyword that is used for casting. Here is the equivalent Kotlin code.

fun todoTask8(expr: Expr): Int {
    when (expr) {
        is Num -> return expr.value
        is Sum -> return todoTask8(expr.left) + todoTask8(expr.right)
        else -> throw IllegalArgumentException("Unknown expression")

As usual, Kotlin is more concise than Java. The when block starts with the when followed by the variable in question. You can have any number of is clauses in this statement followed by the type. The variable is automatically cast to the specified type on the right hand side of the -> operator.

You can click here to see Part 8

Kotlin Koans—Part 7

This portion of the Kotlin Koans showed off a really boss feature of the language: Data Classes. These are special classes whose primary purpose is to hold data.

Here is a Java class that we start with that’s taken directly from the tutorial.

public class Person {
        private final String name;
        private final int age;

        public Person(String name, int age) {
            this.name = name;
            this.age = age;

        public String getName() {
            return name;

        public int getAge() {
            return age;

It’s not a special class. This is a class with a primary constructor, getters/setters and two private variables. It’s also one of my biggest complaints about the Java language. I can do the same thing in Python like this.

class Person:
    def __init__(self, name=None, age=None):
        self.name = name
        self.age = age

Four lines of code in Python. In all fairness to Java, would could just declare name and age to be public variables, but doing so is not only frowned upon, but many Java libraries look for getter/setter method to access a property of a Java bean. Basically speaking, even though we could allow for public access of a Java property, it’s not really practical at this point.

There is a Java library called Lombok that does a lot to solve this problem.

public class Person {
    private String name;
    private String age;

Lombok has been the solution I have used for most of my projects. It’s not perfect however. For example, I can’t use the @Data annotation to make a read only class. That forces me to use a mix of Lombok annotations or define a stereotype annotation. It’s not a huge problem, but it’s still something to think about.

Kotlin data classes take this to a whole other level. Here is the same class in Kotlin.

data class Person(val name: String, val age: Int)

That’s it! One line of code!!! With this single line of code, we get our two properties, it’s getter methods, hashcode, equals, toString() and a constructor. The class is immutable because the variables are declared with the val keyword. We can make the class mutable by using var instead of val. Finally, we aren’t losing our ability to work with existing Java libaries.

I really don’t see how it can get any better than this. I have used data classes countless times when working with ORM libraries such as hibernate. I’d say 95% of these classes are classes that just hold data and map to a table in the database. Although any IDE can generate constructors, getters/setters, equals, and hashcode, and toString, let’s face it, it’s even better to have this built directly into the language itself.

You can click here to see Part 6