Class delegation with by keyword in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of class delegation example

What is the output of this Kotlin code using class delegation with the by keyword?

Kotlin

interface Printer {
    fun printMessage()
}

class ConsolePrinter : Printer {
    override fun printMessage() {
        println("Printing to console")
    }
}

class DelegatingPrinter(printer: Printer) : Printer by printer

fun main() {
    val printer = ConsolePrinter()
    val delegatingPrinter = DelegatingPrinter(printer)
    delegatingPrinter.printMessage()
}

A Printing to console

B No output

C Runtime exception

D Compilation error

💡 Hint

Remember that the by keyword delegates interface implementation to the given object.

The class DelegatingPrinter delegates the Printer interface to ConsolePrinter. Calling printMessage() on delegatingPrinter calls the method on ConsolePrinter, printing the message.

❓ Predict Output

intermediate

Delegation with method override

What will be printed when running this Kotlin code that uses delegation but overrides one method?

Kotlin

interface Speaker {
    fun speak()
    fun shout()
}

class LoudSpeaker : Speaker {
    override fun speak() = println("Speaking softly")
    override fun shout() = println("SHOUTING")
}

class CustomSpeaker(speaker: Speaker) : Speaker by speaker {
    override fun speak() = println("Custom speaking")
}

fun main() {
    val loudSpeaker = LoudSpeaker()
    val customSpeaker = CustomSpeaker(loudSpeaker)
    customSpeaker.speak()
    customSpeaker.shout()
}

A

Speaking softly
SHOUTING

B

Custom speaking
SHOUTING

C

Custom speaking
Speaking softly

D

SHOUTING
Custom speaking

💡 Hint

Delegation forwards calls except for overridden methods in the delegating class.

The CustomSpeaker overrides speak() to print "Custom speaking". The shout() method is delegated to LoudSpeaker, which prints "SHOUTING".

🔧 Debug

advanced

Identify the error in delegation usage

This Kotlin code tries to delegate an interface but causes a compilation error. What is the cause?

Kotlin

interface Calculator {
    fun add(a: Int, b: Int): Int
}

class SimpleCalculator : Calculator {
    override fun add(a: Int, b: Int) = a + b
}

class Delegator : Calculator by SimpleCalculator() {
    fun subtract(a: Int, b: Int) = a - b
}

fun main() {
    val d = Delegator()
    println(d.add(5, 3))
    println(d.subtract(5, 3))
}

A No error, code runs and prints 8 and 2

B Error because Delegator must implement subtract() from Calculator

C Error because SimpleCalculator() cannot be used directly in delegation

D Error because Delegator does not have a primary constructor

💡 Hint

Delegation only requires implementing interface methods; extra methods are allowed.

The code compiles and runs fine. Delegator delegates Calculator methods to SimpleCalculator and adds its own subtract method. The output is 8 and 2.

❓ Predict Output

advanced

Delegation with multiple interfaces

What is the output of this Kotlin program that delegates two interfaces using separate objects?

Kotlin

interface Writer {
    fun write()
}

interface Reader {
    fun read()
}

class FileWriter : Writer {
    override fun write() = println("Writing to file")
}

class FileReader : Reader {
    override fun read() = println("Reading from file")
}

class FileHandler(writer: Writer, reader: Reader) : Writer by writer, Reader by reader

fun main() {
    val writer = FileWriter()
    val reader = FileReader()
    val handler = FileHandler(writer, reader)
    handler.write()
    handler.read()
}

A Compilation error due to multiple delegation

B

Reading from file
Writing to file

C

Writing to file
Reading from file

D Runtime exception due to ambiguous delegation

💡 Hint

Kotlin supports delegation of multiple interfaces by separate objects.

The class FileHandler delegates Writer to FileWriter and Reader to FileReader. Calling write() and read() prints the respective messages in order.

🧠 Conceptual

expert

Understanding delegation and method resolution

Consider a Kotlin class Master that delegates interface Worker to an object but also overrides one method. Which statement is true about method calls on Master?

A All interface methods are forwarded to the delegate object, ignoring any overrides in Master.

B Delegation disables method overriding; Master cannot override any interface method.

C Master must implement all interface methods explicitly; delegation is only for property access.

D Calls to overridden methods in Master execute Master’s code; other interface methods are forwarded to the delegate object.

💡 Hint

Delegation forwards calls except when the delegating class overrides the method.

In Kotlin, delegation forwards interface method calls to the delegate object unless the delegating class overrides the method, in which case the override is used.