Primary Constructor in Kotlin

Introduction

Primary Constructor 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.

Primary constructor is part of the class header. 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

• Primary constructor is part of the class header.
• Constructor parameters can become properties using val or var.
• An init block runs during object initialization.

Syntax

class User(val id: Int)

Primary Constructor in Kotlin Example Program in Kotlin

class User(val id: Int, var active: Boolean) {
    init {
        println("User object created with id $id")
    }
}

fun main(args: Array<String>) {
    val user = User(101, true)
    println("Active: ${user.active}")
}

Sample Output

User object created with id 101
Active: true

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("User object created with id $id") statement writes a line to the console — this produces part of the sample output below.

3. val user = User(101, true) assigns or updates a value used later in the program.

4. The println("Active: ${user.active}") statement writes a line to the console — this produces part of the sample output below.

5. Primary constructor is part of the class header.

6. 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

• Primary constructor is part of the class header.
• Constructor parameters can become properties using val or var.
• An init block runs during object initialization.
• 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.