Constructors

class Book(val title: String, val author: String)

fun main() {
    val book = Book("Kotlin in Action", "Dmitry Jemerov")
    println("${book.title} by ${book.author}")
}

class Book(val title: String, val author: String) {
    constructor(title: String) : this(title, "Unknown")
}

fun main() {
    val book = Book("Kotlin Basics")
    println("${book.title} by ${book.author}")
}

class Circle(radius: Double) {
    val area: Double = Math.PI * radius * radius
}

fun main() {
    val circle = Circle(2.0)
    println("Area: ${circle.area}")
}

class User(val name: String, val role: String = "member")

fun main() {
    val admin = User("Ana", "admin")
    val user = User("Bo")
    println("${admin.name}: ${admin.role}")
    println("${user.name}: ${user.role}")
}

1. Forgetting that a secondary constructor must delegate to the primary constructor with : this(...) when a primary constructor exists.
2. Duplicating validation logic across multiple secondary constructors instead of putting shared setup in the primary constructor or an init block.
3. Confusing constructor parameters with properties; a plain parameter (no val/var) is not accessible outside the constructor unless stored.

• The primary constructor is declared in the class header, right after the class name.
• Secondary constructors are declared with the constructor keyword inside the class body and must delegate to the primary constructor.
• Constructor parameters without val/var are only usable inside the constructor/init block, not as class members.
• Multiple secondary constructors let a class be created in different ways while sharing common setup logic.