by

Learn class delegation with by.

Class delegation with by

In Kotlin the by keyword lets a class delegate the implementation of an interface to another object. Example: class Service(logger: Logger) : Logger by logger. Here Service implements the Logger interface but forwards all Logger methods to the logger parameter.

This is a concise way to use composition over inheritance. Service can override specific methods if needed and delegate the rest. It enables code reuse without the rigidity of inheritance.

Delegation vs inheritance

Delegation provides composition instead of inheritance. With inheritance a subclass is tightly coupled to its parent; with delegation a class wraps an object and forwards work to it. A class can inherit from only one class but delegate to many interfaces.

Example: class Repo(db: Db) : Db by db, Cache by cache delegates two interfaces to two different objects. Delegation also allows swapping the target at runtime, which inheritance cannot. Injecting a fake object for testing is easier.

Exercise Set up Logger delegation

Define interface Logger { fun log(message: String) }. In class ConsoleLogger : Logger write override fun log(message: String) = println(message). Define class Service(logger: Logger) : Logger by logger. In main create Service(ConsoleLogger()) and call service.log("Running").

Show example solution

interface Logger {
    fun log(message: String)
}

class ConsoleLogger : Logger {
    override fun log(message: String) = println(message)
}

class Service(logger: Logger) : Logger by logger

fun main() {
    val service = Service(ConsoleLogger())
    service.log("Running")
}

Test yourself

What does class Service(logger: Logger) : Logger by logger do?
- Only calls the constructor
- Delegates Logger methods to logger
- Creates a new Logger
- Hides the methods
Show answer

Delegates Logger methods to logger — by logger forwards Logger methods to the logger object.

Complete the delegation declaration

class Service(logger: Logger) : Logger ____ logger

Show answer

class Service(logger: Logger) : Logger by logger

Match the delegation parts

by

Delegation
interface Logger

Contract
class Service

Client
logger

Target

Next lesson: Delegated properties

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