init Blocks and Validation

Part of the Object Oriented Programming section of Coddy's Kotlin journey. Lesson 4 of 57.

An init block runs when an instance is created, right after the constructor parameters are assigned. It is the place for setup code that does not fit in a single property declaration:

class Order(val id: Int, items: List<Int>) {
    val total: Int
    val count: Int

    init {
        total = items.sum()
        count = items.size
        println("created order $id")
    }
}

Inside main:

val o = Order(7, listOf(4, 6, 10))
println("${o.count} items, total ${o.total}")

Output:

created order 7
3 items, total 20

require(condition) { message } checks an argument and throws IllegalArgumentException with the message when it is false. Putting it in init means an invalid instance can never be created:

class Percentage(val value: Int) {
    init {
        require(value in 0..100) { "percentage must be 0-100, was $value" }
    }
}

Inside main:

println(Percentage(42).value)
try {
    Percentage(150)
} catch (e: IllegalArgumentException) {
    println(e.message)
}

Output:

42
percentage must be 0-100, was 150

A class can have several init blocks; they run in order, together with the property initializers, from top to bottom:

class Demo(name: String) {
    val first = "first: $name".also { println(it) }

    init { println("init 1") }

    val second = "second".also { println(it) }

    init { println("init 2") }
}

Inside main:

Demo("x")
println("done")

Output:

first: x
init 1
second
init 2
done

Validation in init keeps the rule next to the data. Code that receives an instance can trust it without checking again:

class Email(val address: String) {
    init {
        require("@" in address) { "not an email: $address" }
    }
}
fun send(to: Email) { /* no need to check the address again */ }

Challenge

Easy

Complete the class Password. Its init block checks, with require, that the text has at least 8 characters (message too short) and contains a digit (message needs a digit), in that order. The property strength is computed in init: strong when the text also contains an upper-case letter, otherwise ok.

The supplied code creates a Password for each input line and prints strength or the error message.

Your code goes in Password.kt. Main.kt holds the supplied input/output code and cannot be edited.

Try it yourself

fun main() {
    // Supplied input/output code: keep it as it is
    val input = generateSequence(::readLine).toList()
    for (line in input) {
        try {
            println(Password(line).strength)
        } catch (e: IllegalArgumentException) {
            println(e.message)
        }
    }
}