Secondary Constructor in Kotlin

Introduction

Secondary 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.

Secondary constructors are declared inside the class body using constructor keyword. 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

• Secondary constructors are declared inside the class body using constructor keyword.
• They must call the primary constructor using this(…).
• Useful when multiple construction patterns are needed.

Syntax

constructor(name: String) : this(name, 0)

Secondary Constructor in Kotlin Example Program in Kotlin

class Employee(val name: String, val id: Int) {
    constructor(name: String) : this(name, 0) {
        println("Secondary constructor used")
    }
}

fun main(args: Array<String>) {
    val emp = Employee("Ravi")
    println("${emp.name} -> ${emp.id}")
}

Sample Output

Secondary constructor used
Ravi -> 0

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("Secondary constructor used") statement writes a line to the console — this produces part of the sample output below.

3. val emp = Employee("Ravi") assigns or updates a value used later in the program.

4. The println("${emp.name} -> ${emp.id}") statement writes a line to the console — this produces part of the sample output below.

5. Secondary constructors are declared inside the class body using constructor keyword.

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

• Secondary constructors are declared inside the class body using constructor keyword.
• They must call the primary constructor using this(…).
• Useful when multiple construction patterns are needed.
• 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.