Class Constructors in Kotlin

Introduction

Class Constructors 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.

Constructors initialize object properties when an object is created. 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

• Constructors initialize object properties when an object is created.
• Kotlin has primary and secondary constructors.
• Primary constructor is declared in the class header.

Syntax

class Person(val name: String)

Class Constructors in Kotlin Example Program in Kotlin

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

fun main(args: Array<String>) {
    val person = Person("Meena", 22)
    println("${person.name} is ${person.age} years old")
}

Sample Output

Meena is 22 years old

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. val person = Person("Meena", 22) assigns or updates a value used later in the program.

3. The println("${person.name} is ${person.age} years old") statement writes a line to the console — this produces part of the sample output below.

4. Constructors initialize object properties when an object is created.

5. 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

• Constructors initialize object properties when an object is created.
• Kotlin has primary and secondary constructors.
• Primary constructor is declared in the class header.
• 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.