Class Declaration in Kotlin

Introduction

Class Declaration 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.

A class is a blueprint for creating objects. 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

• A class is a blueprint for creating objects.
• Use the class keyword to declare a class.
• Classes can contain properties and functions (methods).

Syntax

class Student { }

Class Declaration in Kotlin Example Program in Kotlin

class Student {
    var name = "Unknown"
    fun display() = println("Student: $name")
}

fun main(args: Array<String>) {
    val student = Student()
    student.name = "Arun"
    student.display()
}

Sample Output

Student: Arun

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 name = "Unknown" assigns or updates a value used later in the program.

3. The println("Student: $name") statement writes a line to the console — this produces part of the sample output below.

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

5. student.name = "Arun" assigns or updates a value used later in the program.

6. A class is a blueprint for creating objects.

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

• A class is a blueprint for creating objects.
• Use the class keyword to declare a class.
• Classes can contain properties and functions (methods).
• 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.