The init Block

class Person(val name: String, val age: Int) {
    init {
        println("Creating person: $name, age $age")
    }
}

fun main() {
    val person = Person("Ana", 30)
    println("Created: ${person.name}")
}

class Age(val value: Int) {
    init {
        require(value >= 0) { "Age cannot be negative" }
    }
}

fun main() {
    val age = Age(25)
    println("Valid age: ${age.value}")
}

class Steps {
    val first = println("Step 1: property initialized").let { "done" }
    init {
        println("Step 2: first init block")
    }
    val second = println("Step 3: second property initialized").let { "done" }
    init {
        println("Step 4: second init block")
    }
}

fun main() {
    Steps()
}

class Logger(val prefix: String) {
    init {
        println("[$prefix] Logger initialized")
    }

    constructor() : this("DEFAULT") {
        println("Secondary constructor body running")
    }
}

fun main() {
    Logger()
}

1. Forgetting that multiple init blocks and property initializers run in the exact order they are written in the class body, not in a separate fixed order.
2. Putting logic in init that depends on a secondary constructor's parameter, forgetting init blocks run before secondary constructor bodies (but after the primary constructor's own delegation).
3. Duplicating validation logic in every secondary constructor instead of placing shared checks in an init block once.

• init blocks run as part of instance creation, in the order they appear alongside property initializers in the class body.
• A class can have multiple init blocks, and they execute top-to-bottom interleaved with property initialization.
• init blocks can access primary constructor parameters directly, since they run as part of primary construction.
• Validation logic common to every instance is often placed in an init block rather than repeated per constructor.