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Kotlin: An Illustrated Guide • Chapter 4

Introduction to Classes and Objects

Chapter cover image

Way back in chapter 1, we learned about a variety of types in Kotlin, such as Double, String, and Boolean. In this chapter, we’re going to start creating our very own types.

Putting Variables and Functions Together

As you might recall, we started off this book creating variables to hold things pertaining to a circle, such as its radius and circumference. And in chapter 2, we created a function to calculate the circumference from the radius.

It all looked kind of scrambled, like this:

Lots of radius and circumference variables for circles of different sizes

This was manageable for such a simple example, but as we come up with more and more things that we want to know about the circle, such as its diameter, area, or position, it can become quite difficult to manage all of those different variables and functions. And once we start introducing other shapes, like rectangles and triangles, it becomes even harder to keep things straight - for example, which functions give us the area of a circle, and which give us the area of a rectangle?

So in this chapter, we’ll create a new type called a Circle. This way, instead of having separate variables and functions that hold a radius and a circumference, we can have a single variable that represents the circle itself.

So instead of the scrambled variables and functions above, it’ll look more like this:

The same variables, but with all of the related variables grouped up according to the circle that it represents.

In Kotlin, we can put related variables and functions together using a class, which is a feature that we will use to create our new Circle type.

Defining a Class

Let’s create our very first class, a Circle, which will include these variables and functions:

  • The radius variable
  • The read-only pi variable
  • The circumference() function

Instead of jumping into all of this at once, let’s build up this class slowly… one step at a time… and look carefully at each part.

An Empty Class

First, let’s define an empty class - with no variables and no functions.

class Circle

This is all it takes to create a new class called Circle!

When we create a class, we’re creating a new type, just like Int, Double, and String. In other words, once we define the Circle class like this, we can use it anywhere that we’d normally put a type, such as a function parameter.

fun draw(circle: Circle) {
  // Code that draws the circle would go here

Our First Diagram

It’s often useful to diagram your classes, so that you can visualize them and communicate your ideas to your friends. We’ll also use diagrams throughout the rest of this book to help explain concepts.

So, as we’re building out this Circle class, we’ll also diagram it at each stage, using a diagramming standard called the Unified Modeling Language, or UML for short.

We’ll start simple. To show a class, just put the name of the class in a box, like this. (I’ll also add the Kotlin code alongside the diagram, so that you can compare them).

A very simple UML class diagram for an empty class called Circle
class Circle

Now that we’ve got our new Circle type, we could create a variable to hold it, but since our class is completely empty, it’s not particularly useful yet. So, before we do that, let’s give our Circle a radius!

Adding a radius Property

In real life, every circle has a radius, so we need to make sure that every Circle in our code also has a radius variable. Here’s how we can do that:

class Circle(var radius: Double)

Here, we added a new variable to the class, called radius, which has a type of Double. The value of radius can be changed, because of the var that comes before it. In Kotlin, a variable in a class like this is called a property of the class.

Let’s also update our diagram to show that a Circle class has a radius property. To do this, we draw a horizontal line under the name of the class, and write out the name of the property and its type, almost identically to how we write it in our Kotlin code:

UML class diagram for a Circle with a single property called radius
class Circle(var radius: Double)

Now that we’ve got a circle class with a radius, we’re ready to start using it!


In this book, we’ve already created lots of variables. For example, here’s how we can declare and assign a variable with a Double type:

val radiusOfSmallCircle: Double = 5.2

As mentioned, when we created the class, we made a new type called Circle. Just like you can have a variable that’s a Double type, you can also have a variable that’s a Circle type. Declaring and assigning a Circle variable is easy:

val smallCircle = Circle(5.2)

This creates a new variable called smallCircle, which is assigned a Circle with a radius of 5.2.

Keep in mind - just as you can have many different Double values like…

  • 5.2
  • 6.7
  • 10.0

… you can also have lots of different Circle values like…

  • A circle with a radius of 5.2
  • A circle with a radius of 6.7
  • A circle with a radius of 10.0

But when it comes to classes, instead of calling these values, we usually call them objects.

Constructing Objects

Let’s look at that code again.

// Declaring the class
class Circle(var radius: Double)

// Using the class
val smallCircle = Circle(5.2)

What’s happening when we write Circle(5.2)?

It’s kind of like we’re calling a function named Circle() that has a parameter called radius and a return type of Circle. These kinds of functions aren’t called functions, though. They’re called constructors because they construct a new object.

Constructor Parameters

Note that when you call a constructor, you must provide an argument for every property that is listed between the constructor’s opening and closing parentheses - ( and ). Since we put var radius: Double between those parentheses, we have to provide an argument of type Double when we call the constructor:

The 5.2 argument is passed to the constructor as the radius parameter

Note that radius is actually filling two roles:

  1. It’s a constructor parameter. You have to provide a value for it whenever you call the constructor. (However, just like with function parameters, you can also give a constructor parameter a default argument).
  2. It’s a property. You’ll be able to get the value of the radius from any circle object. We’ll see an example of this in a moment.

A constructor parameter like radius, which is preceded by val or var, is sometimes also called a property parameter, because it acts as both a property and a constructor parameter.

When you create an object, we say that you’re creating an instance of the class. For that reason, creating an object is sometimes referred to as instantiating the class.

Classes vs. Objects

The difference between classes and objects can be confusing at first, so let’s take a moment to clarify it.

A class describes the characteristics and behavior of some concept. If we’re talking about circles, those characteristics might include its radius, diameter, circumference, and area.

An object is an actual particular instance of that thing. Here are three circle _objects~:

Circles of different sizes

A circle class answers questions like:

  • “What does it mean for something to be a circle?”
  • “What characteristics does it have?”
  • “What does it do?”

A circle object answers questions like:

  • “What is the radius of this particular circle?”
  • “What is its circumference?”
  • “What is its area?”

Here are a few more examples to help distinguish between classes and objects.

  • You might have a Number class, with objects like 32,768 or 6.62607015.
  • You might have a Color class, with objects like red, green, and blue.
  • You might have a Dog class, with objects like Fido, Rover, or Mrs. Wagglytails.
Multiple dogs, representing multiple objects of the same class.

We’ll see plenty more examples of classes and objects throughout the rest of this book!

Getting a Property’s Value

Now that we’ve created a circle object, how can we get its radius?

Easy: to get the value of a property on an object, type the name of the variable, a dot ., and the name of the property, like this:

val smallCircle = Circle(5.2)
val radiusOfSmallCircle: Double = smallCircle.radius

After running that code, radiusOfSmallCircle will be equal to 5.2.

That takes care of radius. It’s time to move onto the other property, pi.

Constant Properties

We could put pi inside the parentheses like we did for radius, separating them with a comma:

class Circle(var radius: Double, val pi: Double)

But if we do this, then we’d have to provide pi every time we instantiate a circle, like this:

val smallCircle = Circle(5.2, 3.14)
val mediumCircle = Circle(6.7, 3.14)
val largeCircle = Circle(10.0, 3.14)

Hmm… that’s not quite what we want. Since pi should always be the exact same value regardless of the particular circle, it doesn’t make sense for it to be a constructor parameter.

In fact, it would be better if the calling code could never specify the value of pi when constructing a Circle.

To do that, we can simply move pi out of the parentheses, so that it looks like this:

class Circle(var radius: Double) {
  val pi: Double = 3.14

Here, we added an opening brace { and a closing brace }. Everything between those braces is called the body of the class. Inside of the body, we declare pi, and assign it a value of 3.14.

By moving it out of the parentheses, the pi property is no longer a constructor parameter, so we can continue to call the constructor with just the radius, as we did back in Listing 4.5:

val smallCircle = Circle(5.2)

Private Properties

As it is now, you can take any circle and get the value of both radius and pi:

val smallCircle = Circle(5.2)

val radiusOfSmallCircle = smallCircle.radius
val piFromSmallCircle = smallCircle.pi

I can’t think of too many reasons why code outside of the class would need the value of pi. Let’s make it so that this property is only visible from inside the class. To do that, we can add the keyword private when we declare it.

class Circle(var radius: Double) {
  private val pi: Double = 3.14

Now, if you try to get the value of the pi property from outside of the class body, you’ll get an error:

val smallCircle = Circle(5.2)

val radiusOfSmallCircle = smallCircle.radius
val piFromSmallCircle = smallCircle.pi

Let’s update our UML class diagram to include the pi property. We can indicate the visibility of a property in the diagram:

  • Private properties are preceded with a - symbol
  • Public properties are preceded with a + symbol
UML diagram for a Circle with two properties, including visibility indicators
class Circle(var radius: Double) {
    private val pi: Double = 3.14

Now, we’re ready to add the circumference() function to the class!

Adding a Member Function

When a function belongs to a class, it’s often called a method or member function. Adding a method to a class is easy - just put it into the body of the class. To start with, let’s just take the exact same function from Listing 2.3, and plop it verbatim into our Circle class:

class Circle(var radius: Double) {
  private val pi: Double = 3.14

  fun circumference(radius: Double) = 2 * pi * radius

When first we created the circumference() function back in Chapter 2, it made sense for it to have a parameter called radius. But now that we’re adding this function to the class, we can just refer to the value of the radius property instead.

In other words, instead of referring to the radius parameter, like this…

The radius inside the function body refers to the radius argument of the circumference function.

… we can remove the radius parameter from the function, so that it refers to the property, like this…

The radius inside the function body refers to the radius argument of the circumference function.

This introduces the concept of scope, which we will explore in depth in an upcoming chapter. For now, just be sure to remove the parameter from the circumference() function so that 2 * pi * radius will refer to the radius property.

class Circle(var radius: Double) {
  private val pi: Double = 3.14

  fun circumference() = 2 * pi * radius

Now that Circle has a circumference() function, how can we call it?

Calling a function on an object is done similarly to how we got the value of radius: the name of the variable, a dot ., and the name of the function.

val smallCircle = Circle(5.2)
val circumferenceOfSmallCircle: Double = smallCircle.circumference()

And before we move on, let’s add circumference() to our diagram!

UML class diagram with two properties and a function.
class Circle(var radius: Double) {
  private val pi: Double = 3.14

  fun circumference() = 2 * pi * radius

Note that the diagram does not include the body of the function. In other words, 2 * pi * radius does not appear in it. That’s because class diagrams are designed to give you an idea of what data and behavior are a part of the class, without going into the specifics.

Adding More Functions

When we started off this chapter, we only had a radius variable and a circumference() function. Now that we’ve put those two things together into a class, it’s time to fill out our Circle class with other things that you might want to know about a circle.

For example, we can easily add a function that calculates the area of the circle.

class Circle(var radius: Double) {
  private val pi: Double = 3.14

  fun circumference() = 2 * pi * radius
  fun area() = pi * radius * radius

val smallCircle = Circle(5.2)
val areaOfSmallCircle = smallCircle.area()

We can also add a function to calculate its diameter.

class Circle(var radius: Double) {
  private val pi: Double = 3.14

  fun circumference() = 2 * pi * radius
  fun area() = pi * radius * radius
  fun diameter() = 2 * radius

val smallCircle = Circle(5.2)
val diameterOfSmallCircle = smallCircle.diameter()

We can even change how we calculate the circumference, by using the diameter() function instead:

class Circle(var radius: Double) {
  private val pi: Double = 3.14

  fun circumference() = diameter() * pi
  fun area() = pi * radius * radius
  fun diameter() = 2 * radius

And now, we can add these last few functions to our UML diagram.

UML class diagram for a Circle with multiple properties and functions
class Circle(var radius: Double) {
  private val pi: Double = 3.14

  fun circumference() = diameter() * pi
  fun area() = pi * radius * radius
  fun diameter() = 2 * radius

Anatomy of a Class

Now that we’ve covered the basics of Kotlin classes, here’s a recap of the main pieces:

Anatomy of a class in Kotlin

Everything is an Object

Up until this chapter, we’ve only used built-in Kotlin types, such as Double, String, and Boolean. You might be surprised to learn that when we used those types, we were actually using classes and objects! Just like we used the dot . to get properties and call functions on our Circle objects, we can also use a dot on any of these types.

Doubles as Objects

For example, objects of type Double have functions like plus() and times(). So instead of writing our circumference() function like this…

fun circumference() = 2 * pi * radius

… we can instead write it like this …

fun circumference() = 2.times(pi).times(radius)

Kotlin developers normally use the arithmetic operators (+, -, *, /) in most cases, but the functions are there if you want them!

Strings as Objects

String objects also have some interesting properties and functions. Here are a few examples:

  • The length property tells you how many characters (i.e., letters, numbers, and symbols) are in the string.
  • toUpperCase() will force all of the letters in the string to upper case.
  • drop() will remove characters from the beginning of the string.

Here’s how that looks in code:

val greeting: String = "Welcome"

val numberOfLettersInGreeting = greeting.length // Evaluates to 7
val loudGreeting = greeting.toUpperCase()       // Evaluates to "WELCOME"
val substring = greeting.drop(3)                // Evaluates to just "come"

Booleans as Objects

Even Boolean variables - which are only ever true or false - are objects! For example, if you want to turn on the headlights of your car if either it’s dark or it’s raining, you can write code to do that like this:

val isDark: Boolean = true
val isRaining: Boolean = false

val shouldTurnOnHeadlights = isDark.or(isRaining)
val shouldStayHome = isDark.and(isRaining)

As you can see, objects are everywhere in Kotlin! Even simple values are objects.


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Classes are powerful, and even though we covered a lot of ground in this chapter, we’ve really only introduced them. They open up an entire new world of ways to represent your concepts in code. Later in this book, we’ll cover more advanced concepts like abstraction and inheritance. But for now, you should be proud of your progress!

In this chapter, we finally created our very own types, using classes. We learned:

  • How to define a class
  • How to create an object - that is, an instance of a class.
  • How to add properties to a class, and how to access them.
  • How to add functions to a class, and how to call them.
  • How to create a UML diagram for a single class.
  • How to call functions and get properties from the built-in Kotlin types that we’ve already worked with, such as Double, String, and Boolean.

And… just in case you haven’t had enough, you also can see a wide range of examples of classes in Kotlin.

In the next chapter, we’ll look at a special kind of class, called an enum class. See you then!

Thanks to Tobenna Ezike and Esraa Ibrahim for reviewing this chapter.

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