Class delegation with by keyword in Kotlin

Introduction

Class delegation with the by keyword lets one class use another class's behavior easily. It helps avoid repeating code by passing work to another class.

When you want one class to reuse another class's functions without rewriting them.

When you want to add extra features to a class but keep the original behavior.

When you want to split responsibilities between classes but keep a simple interface.

When you want to change how some methods work but keep others the same.

When you want to follow the rule 'favor composition over inheritance' in your design.

Syntax

Kotlin

class ClassName(val delegate: InterfaceType) : InterfaceType by delegate

The class ClassName promises to implement InterfaceType.

Instead of writing all methods, it uses by delegate to forward calls to delegate.

Examples

This example shows a class DelegatedPrinter that uses ConsolePrinter to do the printing work.

Kotlin

interface Printer {
    fun print()
}

class ConsolePrinter : Printer {
    override fun print() {
        println("Printing on console")
    }
}

class DelegatedPrinter(val printer: Printer) : Printer by printer

This example adds counting behavior by overriding the print method but still delegates other methods.

Kotlin

class CountingPrinter(val printer: Printer) : Printer by printer {
    var count = 0
    override fun print() {
        count++
        println("Print called $count times")
        printer.print()
    }
}

Sample Program

This program shows how BilingualSpeaker uses delegation to speak English or Spanish depending on the delegate passed.

Kotlin

interface Speaker {
    fun speak()
}

class EnglishSpeaker : Speaker {
    override fun speak() {
        println("Hello!")
    }
}

class SpanishSpeaker : Speaker {
    override fun speak() {
        println("¡Hola!")
    }
}

class BilingualSpeaker(private val speaker: Speaker) : Speaker by speaker

fun main() {
    val english = EnglishSpeaker()
    val spanish = SpanishSpeaker()

    val bilingualEnglish = BilingualSpeaker(english)
    val bilingualSpanish = BilingualSpeaker(spanish)

    bilingualEnglish.speak()
    bilingualSpanish.speak()
}

Important Notes

Delegation helps keep code clean and easy to maintain.

You can override some methods in the delegating class to add or change behavior.

Delegation works only with interfaces or abstract classes in Kotlin.

Summary

Class delegation with by lets one class reuse another's behavior easily.

It helps avoid repeating code and keeps classes focused on their job.

You can add extra features by overriding delegated methods.

Practice

1. What does the by keyword do in Kotlin class delegation?

easy

A. It overrides all methods of a class.

B. It creates a new instance of a class.

C. It marks a class as abstract.

D. It lets a class reuse another class's implementation automatically.

Solution

1. Step 1: Understand the role of by keyword

   The by keyword in Kotlin is used for class delegation, which means one class can reuse the implementation of another class without rewriting the code.

2. Step 2: Identify what by does

   It automatically forwards calls to the delegated class, so the delegating class behaves like the delegated one.

3. Final Answer:

   It lets a class reuse another class's implementation automatically. -> Option D

4. Quick Check:

   Delegation = reuse implementation [OK]

Hint: Remember: by means reuse another class's code [OK]

Common Mistakes:

• Thinking by creates a new instance
• Confusing by with inheritance
• Assuming by overrides methods automatically

2. Which of the following is the correct syntax to delegate interface Printer to class ConsolePrinter in Kotlin?

easy

A. class MyPrinter : Printer by ConsolePrinter()

B. class MyPrinter implements Printer by ConsolePrinter()

C. class MyPrinter extends Printer by ConsolePrinter()

D. class MyPrinter delegate Printer to ConsolePrinter()

Solution

1. Step 1: Recall Kotlin delegation syntax

   In Kotlin, delegation uses the syntax: class ClassName : InterfaceName by instance.

2. Step 2: Match the correct syntax

   class MyPrinter : Printer by ConsolePrinter() matches this pattern exactly: class MyPrinter : Printer by ConsolePrinter().

3. Final Answer:

   class MyPrinter : Printer by ConsolePrinter() -> Option A

4. Quick Check:

   Syntax = class X : Interface by instance [OK]

Hint: Use colon and 'by' for delegation syntax [OK]

Common Mistakes:

• Using 'implements' or 'extends' keywords (Java style)
• Writing 'delegate' keyword which doesn't exist
• Missing colon before interface name

3. What is the output of this Kotlin code?

interface Logger {
    fun log(msg: String)
}

class ConsoleLogger : Logger {
    override fun log(msg: String) {
        println("Console: $msg")
    }
}

class AppLogger(logger: Logger) : Logger by logger

fun main() {
    val logger = AppLogger(ConsoleLogger())
    logger.log("Hello")
}

medium

A. No output

B. Console: Hello

C. Error: log() not implemented

D. Hello

Solution

1. Step 1: Understand delegation in AppLogger

   The class AppLogger delegates Logger interface to the passed logger instance, which is ConsoleLogger.

2. Step 2: Trace the log call

   Calling logger.log("Hello") calls ConsoleLogger.log, which prints "Console: Hello".

3. Final Answer:

   Console: Hello -> Option B

4. Quick Check:

   Delegation calls delegated method output [OK]

Hint: Delegated methods run from the delegated instance [OK]

Common Mistakes:

• Expecting just 'Hello' without prefix
• Thinking delegation requires manual override
• Assuming no output because of delegation

4. Identify the error in this Kotlin code using class delegation:

interface Speaker {
    fun speak()
}

class Human : Speaker {
    override fun speak() {
        println("Hello")
    }
}

class Robot(speaker: Speaker) : Speaker {
    speak()
}

medium

A. Missing delegation with 'by' keyword in Robot class.

B. Robot class should not implement Speaker interface.

C. Human class must not override speak().

D. speak() method call is correct as is.

Solution

1. Step 1: Check Robot class declaration

   Robot implements Speaker but does not delegate or implement speak() method itself.

2. Step 2: Identify missing delegation

   Robot should delegate Speaker to the passed speaker instance using by speaker to avoid error.

3. Final Answer:

   Missing delegation with 'by' keyword in Robot class. -> Option A

4. Quick Check:

   Delegation needs 'by' keyword [OK]

Hint: Delegate with 'by' or implement all methods [OK]

Common Mistakes:

• Calling method directly without delegation
• Forgetting to use 'by' keyword
• Assuming interface methods auto-implement

5. You want to create a class SmartSpeaker that delegates Speaker interface to a Human instance but adds a new method playMusic(). Which is the best way to do this?

hard

A. class SmartSpeaker : Human() {
fun playMusic() = println("Playing music")
}

B. class SmartSpeaker : Speaker {
val human = Human()
override fun speak() = human.speak()
fun playMusic() = println("Playing music")
}

C. class SmartSpeaker(human: Human) : Speaker by human {
fun playMusic() {
println("Playing music")
}
}

D. class SmartSpeaker(human: Human) : Speaker {
fun playMusic() = println("Playing music")
}

Solution

1. Step 1: Understand delegation with added methods

   Using by human delegates all Speaker methods to Human automatically.

2. Step 2: Add new method in delegating class

   We can add playMusic() directly in SmartSpeaker without affecting delegation.

3. Final Answer:

   class SmartSpeaker(human: Human) : Speaker by human { fun playMusic() { println("Playing music") } } -> Option C

4. Quick Check:

   Delegate with 'by' and add methods [OK]

Hint: Delegate with 'by' and add new methods in class body [OK]

Common Mistakes:

• Not delegating interface methods properly
• Extending class instead of delegating interface
• Forgetting to implement interface methods