In Kotlin, an init block runs setup logic when a class instance is created.

It is commonly used to validate constructor arguments, initialize derived properties, or perform other construction-time setup.

class User(val name: String, val age: Int) {

    init {
        require(name.isNotBlank()) {
            "Name must not be blank"
        }

        require(age >= 0) {
            "Age must be non-negative"
        }

        println("Created user: $name")
    }
}

Usage:

val user = User("Alice", 30)

When User("Alice", 30) is called, the init block runs automatically.

Key points

• init blocks are part of the class initialization process.
• They run after primary constructor parameters are available.
• A class can have multiple init blocks.
• Multiple init blocks run in the order they appear in the class body.
• They are especially useful with primary constructors.

Example with multiple init blocks:

class Rectangle(val width: Int, val height: Int) {

    val area: Int

    init {
        require(width > 0) {
            "Width must be positive"
        }
    }

    init {
        require(height > 0) {
            "Height must be positive"
        }

        area = width * height
    }
}

You can also combine init blocks with property initializers:

class Product(val price: Double) {

    val tax = price * 0.2

    init {
        require(price >= 0) {
            "Price cannot be negative"
        }
    }
}

Initialization happens in the order the declarations appear in the class body, so property initializers and init blocks are executed top to bottom.