1. Factory Method · Design Patterns

[THEORY]

[PREDICT]

Read the code carefully before you run it.

prediction.kt

interface Notifier { fun notify(user: String, message: String) }

class EmailNotifier : Notifier {
    override fun notify(user: String, message: String) = println("Email to " + user + ": " + message)
}

fun notifierFor(channel: String): Notifier = when (channel) {
    "email" -> EmailNotifier()
    else -> throw IllegalArgumentException("Unknown channel: " + channel)
}

fun main() {
    notifierFor("email").notify("lena", "Welcome")
    notifierFor("sms").notify("omar", "Ping")
}

What does this program print? And where would you make the change to support the sms channel, so that main stays exactly as it is?

[TASK]

Finish the factory. notifierFor must return an EmailNotifier for the channel email and an SmsNotifier for sms, and throw IllegalArgumentException for any other channel. Each notifier's format(user, message) returns channel|user|message, for example email|lena|Welcome. The caller must never name a concrete class itself. Then use notifierFor("email") and print the formatted message in main.

task.kt

// TODO: finish the factory. `notifierFor` must return an `EmailNotifier` for // "email" and an `SmsNotifier` for "sms", and throw IllegalArgumentException // for any other channel. The caller must never name a concrete class itself. interface Notifier { fun format(user: String, message: String): String } class EmailNotifier : Notifier { override fun format(user: String, message: String): String { // TODO: return "email|" + user + "|" + message TODO() } } class SmsNotifier : Notifier { override fun format(user: String, message: String): String { // TODO: return "sms|" + user + "|" + message TODO() } } fun notifierFor(channel: String): Notifier { // TODO: return the matching Notifier, or throw for an unknown channel. TODO() } fun main() { // TODO: use notifierFor("email") and print the formatted message. }

Before you get a hint: what have you tried, and where exactly are you stuck?

[COMPREHENSION CHECK]

1. What is the main benefit of a factory method?

It removes the need for a separate product interface altogether It moves the concrete-class choice away from the callers It makes object creation run faster at each call site It keeps exactly one shared instance of every product

2. When is a factory method likely over-engineering?

When there is exactly one product type and no sign of another When the constructor takes more than two arguments in total When the product implements an interface with several methods When the class is used in more than one module of the app

3. What does the calling code depend on when it uses a factory?

The internal private fields of each product it receives Every single concrete product class that it might possibly receive The constructor of each concrete product class it uses The product interface and the factory, not the concrete classes