Delegation vs inheritance decision in Kotlin

Introduction

Delegation and inheritance are ways to share behavior between classes. Choosing the right one helps keep code simple and easy to change.

When you want to reuse code but keep classes loosely connected.

When you want to change behavior without changing the original class.

When you want to avoid deep or complex class hierarchies.

When you want to add features to a class without modifying it.

When you want to share behavior between unrelated classes.

Syntax

Kotlin

open class Parent {
    // parent class example
}

class Child : Parent() {
    // inheritance example
}

interface SomeInterface {
    fun doSomething()
}

class Delegator(private val delegate: SomeInterface) : SomeInterface by delegate {
    // delegation example
}

Inheritance uses a class extending another class.

Delegation uses a class holding another object and forwarding calls to it.

Examples

Inheritance example: Dog inherits from Animal and changes the sound.

Kotlin

open class Animal {
    open fun sound() = "Some sound"
}

class Dog : Animal() {
    override fun sound() = "Bark"
}

Delegation example: SoundDelegator forwards sound() calls to the Cat instance.

Kotlin

interface SoundMaker {
    fun sound(): String
}

class Cat : SoundMaker {
    override fun sound() = "Meow"
}

class SoundDelegator(private val soundMaker: SoundMaker) : SoundMaker by soundMaker

Sample Program

This program shows inheritance with ColorPrinter extending Printer, and delegation with MultiFunctionDevice forwarding scan() to SimpleScanner and print() to ColorPrinter.

Kotlin

open class Printer {
    open fun print() = "Printing document"
}

class ColorPrinter : Printer() {
    override fun print() = "Printing document in color"
}

interface Scanner {
    fun scan(): String
}

class SimpleScanner : Scanner {
    override fun scan() = "Scanning document"
}

class MultiFunctionDevice(private val printer: Printer, private val scanner: Scanner) : Scanner by scanner {
    fun print() = printer.print()
}

fun main() {
    val colorPrinter = ColorPrinter()
    val simpleScanner = SimpleScanner()
    val device = MultiFunctionDevice(colorPrinter, simpleScanner)
    println(device.print())
    println(device.scan())
}

Important Notes

Use inheritance when classes have a clear "is-a" relationship.

Use delegation to share behavior without tight coupling.

Delegation helps avoid problems with deep inheritance trees.

Summary

Inheritance means one class extends another and inherits its behavior.

Delegation means one class uses another class to do some work.

Choose delegation to keep code flexible and inheritance for clear "is-a" relationships.

Practice

1. Which statement best describes when to use inheritance in Kotlin?

easy

A. When you want to hide implementation details completely.

B. When you want to reuse code without creating a parent-child relationship.

C. When a class is a specialized type of another class ("is-a" relationship).

D. When you want to avoid any relationship between classes.

Solution

1. Step 1: Understand inheritance purpose

   Inheritance models an "is-a" relationship where a subclass extends a superclass.

2. Step 2: Match the correct use case

   Using inheritance is appropriate when a class is a specialized version of another class.

3. Final Answer:

   When a class is a specialized type of another class ("is-a" relationship). -> Option C

4. Quick Check:

   Inheritance = "is-a" relationship [OK]

Hint: Use inheritance for clear "is-a" relationships [OK]

Common Mistakes:

• Confusing delegation with inheritance
• Using inheritance for code reuse only
• Thinking inheritance hides implementation

2. Which Kotlin syntax correctly shows delegation of interface Printer by class Document?

easy

A. class Document : Printer(printer)

B. class Document(val printer: Printer) : Printer by printer

C. class Document : Printer implements printer

D. class Document(printer: Printer) extends Printer

Solution

1. Step 1: Recall Kotlin delegation syntax

   Kotlin uses : Interface by instance to delegate interface implementation.

2. Step 2: Identify correct syntax

   class Document(val printer: Printer) : Printer by printer correctly shows delegation with class Document(val printer: Printer) : Printer by printer.

3. Final Answer:

   class Document(val printer: Printer) : Printer by printer -> Option B

4. Quick Check:

   Delegation syntax = : Interface by instance [OK]

Hint: Delegation uses ': Interface by instance' syntax [OK]

Common Mistakes:

• Using extends keyword (not Kotlin)
• Writing implements instead of by
• Missing val/var for delegated instance

3. What is the output of this Kotlin code?

interface Logger {
    fun log(message: String)
}

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

class FileLogger : Logger {
    override fun log(message: String) {
        println("File: $message")
    }
}

class Service(logger: Logger) : Logger by logger

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

medium

A. Console: Start

B. File: Start

C. Start

D. Compilation error

Solution

1. Step 1: Understand delegation in Service class

   Service delegates Logger interface to the passed logger instance, here ConsoleLogger.

2. Step 2: Trace the log call

   Calling service.log("Start") calls ConsoleLogger.log, printing "Console: Start".

3. Final Answer:

   Console: Start -> Option A

4. Quick Check:

   Delegation calls ConsoleLogger.log = Console: Start [OK]

Hint: Delegation calls passed instance methods [OK]

Common Mistakes:

• Confusing which logger is used
• Expecting FileLogger output
• Thinking delegation copies methods

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

interface Worker {
    fun work()
}

class Employee : Worker {
    override fun work() {
        println("Employee working")
    }
}

class Manager(worker: Worker) : Worker by worker {
    fun manage() {
        println("Managing team")
    }
}

fun main() {
    val manager = Manager()
    manager.work()
    manager.manage()
}

medium

A. Manager class missing argument for Worker in constructor call.

B. Employee class does not implement work() method.

C. Delegation syntax is incorrect; should use extends instead of by.

D. manage() method cannot be called on Manager instance.

Solution

1. Step 1: Check Manager constructor usage

   Manager requires a Worker parameter in constructor but none is provided in main().

2. Step 2: Identify compilation error

   Calling Manager() without argument causes a compilation error due to missing parameter.

3. Final Answer:

   Manager class missing argument for Worker in constructor call. -> Option A

4. Quick Check:

   Constructor argument missing = compilation error [OK]

Hint: Check constructor parameters when using delegation [OK]

Common Mistakes:

• Ignoring constructor parameters
• Confusing delegation syntax with inheritance
• Assuming manage() is inaccessible

5. You want to design a Kotlin class SmartDevice that can either behave like a Phone or a Camera depending on user choice at runtime. Which approach is best and why?

hard

A. Use static methods in SmartDevice to mimic Phone and Camera features.

B. Use inheritance with SmartDevice extending Phone or Camera based on a flag.

C. Use multiple inheritance to inherit both Phone and Camera directly.

D. Use delegation to delegate Phone or Camera behavior to SmartDevice dynamically.

Solution

1. Step 1: Analyze design needs

   SmartDevice needs to switch behavior dynamically between Phone and Camera at runtime.

2. Step 2: Choose delegation for flexibility

   Delegation allows SmartDevice to hold and switch between Phone or Camera instances easily without inheritance constraints.

3. Step 3: Exclude other options

   Inheritance is static and cannot switch base class at runtime; Kotlin does not support multiple inheritance; static methods do not provide instance behavior.

4. Final Answer:

   Use delegation to delegate Phone or Camera behavior to SmartDevice dynamically. -> Option D

5. Quick Check:

   Dynamic behavior switching = delegation [OK]

Hint: Use delegation for flexible runtime behavior changes [OK]

Common Mistakes:

• Trying to use inheritance for runtime behavior switch
• Assuming Kotlin supports multiple inheritance
• Using static methods instead of instance delegation