Kotlin Classes Basics

Classes are the foundation of object-oriented programming in Kotlin. They serve as blueprints for creating objects, which combine data and behavior in a single unit. In this tutorial, we'll explore the basics of Kotlin classes, including how to define them, create objects, work with properties, and understand constructors.

What Are Classes?

In Kotlin, a class is a blueprint that defines the structure and behavior of objects. Think of a class as a template that describes:

• What data the object will store (properties)
• What actions the object can perform (methods)
• How the object is created (constructors)

Defining Your First Class

Let's start with a simple class definition:

kotlin

class Person {
    // Class body
}

This creates a basic class named Person. Even this minimal class definition is useful - Kotlin automatically provides:

• A default constructor with no parameters
• Memory allocation for creating instances
• Basic functions like toString(), equals(), and hashCode()

Creating Objects (Instances)

Once you've defined a class, you can create objects (instances) of that class:

kotlin

fun main() {
    val person1 = Person() // Creates a new Person instance
    val person2 = Person() // Creates another Person instance
    
    println(person1)
    println(person2)
}

Output:

Person@1a407d53
Person@3d8c7aca

Notice that Kotlin doesn't use the new keyword for object creation, unlike Java or C#.

Adding Properties to a Class

Properties represent the data each object will store:

kotlin

class Person {
    var name: String = "Unknown"
    var age: Int = 0
}

fun main() {
    val person = Person()
    
    // Accessing properties
    println("Default name: ${person.name}")
    println("Default age: ${person.age}")
    
    // Modifying properties
    person.name = "Alice"
    person.age = 25
    
    println("Updated name: ${person.name}")
    println("Updated age: ${person.age}")
}

Output:

Default name: Unknown
Default age: 0
Updated name: Alice
Updated age: 25

Property Types in Kotlin

In Kotlin, properties can be:

1. Mutable (var): Can be changed after initialization
2. Immutable (val): Can only be assigned once

kotlin

class User {
    val id: Int = 1           // Read-only property (cannot be changed)
    var username: String = "" // Mutable property (can be changed)
}

Constructors

Constructors are special methods used to initialize objects when they're created. Kotlin has two types of constructors:

Primary Constructor

The primary constructor is part of the class header:

kotlin

class Person(val name: String, var age: Int) {
    // Class body
}

fun main() {
    val person = Person("Bob", 30)
    println("Name: ${person.name}, Age: ${person.age}")
    
    // We can change age because it's a var
    person.age = 31
    // We can't change name because it's a val
    // person.name = "Robert" // This would cause a compilation error
    
    println("Updated age: ${person.age}")
}

Output:

Name: Bob, Age: 30
Updated age: 31

Notice how parameters in the primary constructor can become properties by using val or var.

Initialization Blocks

You can add initialization code using init blocks:

kotlin

class Person(val name: String, var age: Int) {
    
    init {
        println("Person created with name: $name")
        
        // Validate age
        require(age >= 0) { "Age cannot be negative" }
    }
}

fun main() {
    val person = Person("Charlie", 25)
    // This would throw an exception: val invalid = Person("Invalid", -1)
}

Output:

Person created with name: Charlie

Secondary Constructors

You can define additional constructors if needed:

kotlin

class Person(val name: String, var age: Int) {
    
    var email: String = ""
    
    // Secondary constructor
    constructor(name: String, age: Int, email: String) : this(name, age) {
        this.email = email
    }
}

fun main() {
    val person1 = Person("Dave", 40)
    val person2 = Person("Emma", 35, "[email protected]")
    
    println("Person 1: ${person1.name}, ${person1.age}, ${person1.email}")
    println("Person 2: ${person2.name}, ${person2.age}, ${person2.email}")
}

Output:

Person 1: Dave, 40, 
Person 2: Emma, 35, [email protected]

Member Functions

Classes can contain functions (methods) that represent the behaviors of objects:

kotlin

class Person(val name: String, var age: Int) {
    
    fun introduce() {
        println("Hi, I'm $name and I'm $age years old")
    }
    
    fun celebrateBirthday() {
        age++
        println("$name just turned $age")
    }
}

fun main() {
    val person = Person("Frank", 45)
    person.introduce()
    person.celebrateBirthday()
}

Output:

Hi, I'm Frank and I'm 45 years old
Frank just turned 46

Real-World Example: Creating a Book Class

Let's create a practical example of a Book class that might be used in a digital library system:

kotlin

class Book(
    val title: String,
    val author: String,
    val year: Int,
    val isbn: String,
    var available: Boolean = true
) {
    // Track number of times this book was borrowed
    private var borrowCount: Int = 0
    
    fun borrow(): Boolean {
        if (available) {
            available = false
            borrowCount++
            println("\"$title\" has been borrowed")
            return true
        }
        println("Sorry, \"$title\" is not available")
        return false
    }
    
    fun returnBook() {
        if (!available) {
            available = true
            println("\"$title\" has been returned")
        } else {
            println("This book wasn't borrowed")
        }
    }
    
    fun displayInfo() {
        println("Book: \"$title\"")
        println("Author: $author")
        println("Year: $year")
        println("ISBN: $isbn")
        println("Available: ${if(available) "Yes" else "No"}")
        println("Times borrowed: $borrowCount")
    }
}

fun main() {
    // Create a few books
    val book1 = Book("The Great Gatsby", "F. Scott Fitzgerald", 1925, "978-3-16-148410-0")
    val book2 = Book("Moby Dick", "Herman Melville", 1851, "978-1-23-456789-7")
    
    // Display initial info
    book1.displayInfo()
    println()
    
    // Borrow and return books
    book1.borrow()
    book2.borrow()
    println()
    
    book1.displayInfo()
    println()
    
    book1.returnBook()
    book1.borrow()
    book1.returnBook()
    println()
    
    // Display final state
    book1.displayInfo()
}

Output:

Book: "The Great Gatsby"
Author: F. Scott Fitzgerald
Year: 1925
ISBN: 978-3-16-148410-0
Available: Yes
Times borrowed: 0

"The Great Gatsby" has been borrowed
"Moby Dick" has been borrowed

Book: "The Great Gatsby"
Author: F. Scott Fitzgerald
Year: 1925
ISBN: 978-3-16-148410-0
Available: No
Times borrowed: 1

"The Great Gatsby" has been returned
"The Great Gatsby" has been borrowed
"The Great Gatsby" has been returned

Book: "The Great Gatsby"
Author: F. Scott Fitzgerald
Year: 1925
ISBN: 978-3-16-148410-0
Available: Yes
Times borrowed: 2

Summary

In this tutorial, you've learned the basics of Kotlin classes:

• How to define classes and create objects
• Working with properties (both mutable and immutable)
• Using primary and secondary constructors
• Adding initialization blocks
• Creating member functions
• A practical example of using classes in a real-world application

Classes are a fundamental building block in Kotlin and object-oriented programming. As you continue your journey, you'll learn more advanced concepts like inheritance, interfaces, and abstract classes that build upon these basics.

Exercises

1. Create a Rectangle class with properties for width and height. Add methods to calculate area and perimeter.

2. Design a BankAccount class with properties for account number, owner name, and balance. Add methods for deposit and withdrawal with appropriate validations.

3. Implement a Student class with properties for name, ID, and a list of grades. Include methods to add grades and calculate the average grade.

Additional Resources

• Official Kotlin Documentation on Classes
• Kotlin Properties and Fields
• Kotlin Constructors

Happy coding with Kotlin classes!