Delegation vs inheritance decision in Kotlin - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of delegation example

What is the output of this Kotlin code using delegation?

Kotlin

interface Printer {
    fun print()
}

class RealPrinter : Printer {
    override fun print() {
        println("Printing from RealPrinter")
    }
}

class Delegator(printer: Printer) : Printer by printer

fun main() {
    val realPrinter = RealPrinter()
    val delegator = Delegator(realPrinter)
    delegator.print()
}

A Printing from RealPrinter

B Runtime exception

C Compilation error

D Printing from Delegator

💡 Hint

Delegation forwards calls to the provided object.

The Delegator class delegates the print() call to RealPrinter, so the output is from RealPrinter.

❓ Predict Output

intermediate

Output of inheritance example

What is the output of this Kotlin code using inheritance?

Kotlin

open class Printer {
    open fun print() {
        println("Printing from Printer")
    }
}

class ChildPrinter : Printer() {
    override fun print() {
        println("Printing from ChildPrinter")
    }
}

fun main() {
    val printer: Printer = ChildPrinter()
    printer.print()
}

A Printing from Printer

B Compilation error

C Runtime exception

D Printing from ChildPrinter

💡 Hint

Inheritance allows overriding methods.

ChildPrinter overrides print(), so calling print() on a ChildPrinter instance prints its own message.

🧠 Conceptual

advanced

Choosing delegation over inheritance

Which scenario best justifies using delegation instead of inheritance in Kotlin?

A When you want to reuse code but avoid tight coupling and allow changing behavior at runtime

B When you want to create a strict type hierarchy with shared implementation

C When you want to override a method in a subclass to change behavior

D When you want to prevent any changes to the base class behavior

💡 Hint

Delegation allows flexible behavior without inheritance constraints.

Delegation allows an object to use another object's functionality without inheriting from it, enabling loose coupling and runtime behavior changes.

❓ Predict Output

advanced

Output with delegation and overridden method

What is the output of this Kotlin code where delegation is combined with an overridden method?

Kotlin

interface Speaker {
    fun speak()
}

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

class CustomSpeaker(private val speaker: Speaker) : Speaker by speaker {
    override fun speak() {
        println("Custom speak start")
        speaker.speak()
        println("Custom speak end")
    }
}

fun main() {
    val english = EnglishSpeaker()
    val custom = CustomSpeaker(english)
    custom.speak()
}

A Compilation error

B

Custom speak start
Hello
Custom speak end

C

Custom speak start
Custom speak end

D Hello

💡 Hint

Overriding a delegated method allows adding behavior before and after delegation.

CustomSpeaker overrides speak() to add prints before and after calling the delegated speaker's speak().

🧠 Conceptual

expert

Why avoid inheritance for behavior reuse?

Why might inheritance be a poor choice for reusing behavior compared to delegation in Kotlin?

A Inheritance cannot override methods, delegation can

B Inheritance is slower at runtime than delegation

C Inheritance creates tight coupling and limits flexibility, while delegation allows composition and runtime behavior changes

D Inheritance requires more code to implement than delegation

💡 Hint

Think about coupling and flexibility differences.

Inheritance tightly couples classes and fixes behavior at compile time, while delegation composes behavior and allows changing it dynamically.

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