KotlinCS 124 LogoJava

Introduction to Objects

class Example(var value: Int)
val example = Example(0)
println(example.value)
example.value = 10
println(example.value)

A new era begins in CS 124. We leave the familiar world of basic types and Strings behind and strike out to new horizons. This lesson begins our discussion of Kotlin objects. Objects represent both a significant conceptual step forward, and dramatically improve our ability to work with data.

So let’s get started!

What Are Objects?
What Are Objects?

Kotlin supports object-oriented programming. What does that mean, exactly?

Wikipedia defines an object as:

In the class-based object-oriented programming paradigm, “object” refers to a particular instance of a class where the object can be a combination of variables, functions, and data structures.

Let’s examine this definition together.

Object Terminology: Class
Object Terminology: Class

A class definines how an entire group of objects behaves. For example, we might say that a person is a class of things where each has a name and an age. Let’s look at our first class definition together:

class Person(var name: String, var age: Int)

Object Terminology: Instance
Object Terminology: Instance

Defining a class allows us to create instances of that class. Sometimes we use the term object and instance interchangeably, defining an object as an instance of a class.

If it helps understand the relationship between class and instance, here are some examples of this relationship between real-world things:

Now, let’s continue the example above using our Person class and create some instances.

class Person(var name: String, var age: Int)

Creating Instances
Creating Instances

To create a new instance of a class we use the classes name in a way that looks like a method call:

class Example(var value: Int)
// Assignment of a new Example instance to the variable e, setting value to 8 initially
val e = Example(8)
println(e.value)

For now the syntax of instance creation will seem somewhat mysterious. Why does it look like a function call? That will make more sense soon. But for now simply follow this template to create new class instances.

Accessing Fields Using Dot Notation
Accessing Fields Using Dot Notation

Objects allow us to create new Kotlin types. These new types are in some ways just like the types that we’ve already been working with:

class Example(var value: Int)
var i = 0 // i is a mutable variable of type int
val e = Example(12) // e is an immutable variable of type Example

But Kotlin classes allow us to include any mixture of different types of data by declaring one or more fields:

// Example has one mutable field named value of type Int
class Example(var value: Int)
// I need to set the field when I create Example instances
val e = Example(8)
// I can access the field value using dot notation:
println(e.value)
e.value = 10
println(e.value)
// Dog has two fields, one named name of type String and another named age of type Double
class Dog(var name: String, var age: Double)
// Again, I can access both Dog fields using dot notation
val d = Dog("Chuchu", 16.1)
println("${d.name} is ${d.age} years old")

Be Patient!
Be Patient!

An important note: not everything about objects is going to make sense to you right away! That’s OK. We’re going to keep practicing, and things will become a bit more clear every day.

More Practice

Need more practice? Head over to the practice page.