init Block in Kotlin

Introduction

init Block 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.

Init block contains initialization code for 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

• init block contains initialization code for a class.
• It runs when an object is created, after primary constructor.
• Multiple init blocks run in the order they appear.

Syntax

init { println("Initialized") }

init Block in Kotlin Example Program in Kotlin

class Product(val name: String) {
    init {
        println("Product object created: $name")
    }
}

fun main(args: Array<String>) {
    Product("Mobile")
}

Sample Output

Product object created: Mobile

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

3. Init block contains initialization code for a class.

4. 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

• init block contains initialization code for a class.
• It runs when an object is created, after primary constructor.
• Multiple init blocks run in the order they appear.
• 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.