Class Properties in Kotlin

Introduction

Class Properties is a fundamental concept every Kotlin developer should understand. Classes bundle data and behaviour. Kotlin reduces boilerplate with concise constructors, properties, and sensible defaults compared to Java.

Properties store state inside a class. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• Properties store state inside a class.
• They can be declared using var (mutable) or val (read-only).
• Properties can have custom getters and setters.

Syntax

var name: String = "Kotlin"

Class Properties in Kotlin Example Program in Kotlin

class Book {
    var title: String = "Kotlin Guide"
    val pages: Int = 250
}

fun main(args: Array<String>) {
    val book = Book()
    println("${book.title} - ${book.pages} pages")
}

Sample Output

Kotlin Guide - 250 pages

When to use

Use classes to model entities with state and behaviour — users, orders, view models, or service objects.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. var title: String = "Kotlin Guide" assigns or updates a value used later in the program.

3. val pages: Int = 250 assigns or updates a value used later in the program.

4. val book = Book() assigns or updates a value used later in the program.

5. The println("${book.title} - ${book.pages} pages") statement writes a line to the console — this produces part of the sample output below.

6. Properties store state inside a class.

7. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Keep constructors short; move complex setup to init blocks or factory functions.
• Expose behaviour through methods rather than public mutable fields.
• Mark classes final (default) unless inheritance is intentional.

Common Mistakes

• Skipping the example and only reading the definition — hands-on practice cements the concept.
• Copying syntax without understanding nullable vs non-nullable types or scope rules.
• Ignoring compiler warnings that often point to safer alternatives.

Key Points

• Properties store state inside a class.
• They can be declared using var (mutable) or val (read-only).
• Properties can have custom getters and setters.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Semicolons at the end of statements are optional in Kotlin.