Why delegation avoids inheritance in Kotlin - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to delegate the print function to the printer object.

Kotlin

class Printer {
    fun print() {
        println("Printing document")
    }
}

class Document(val printer: Printer) {
    fun print() = printer.[1]()
}

Drag options to blanks, or click blank then click option'

AprintDocument

Bprint

Cprintln

Ddisplay

Attempts:

3 left

💡 Hint

Common Mistakes

Using println instead of print causes a compilation error.

Calling a non-existent method like printDocument or display.

The print function in Document delegates the call to the print function of the Printer object.

2fill in blank

medium

Complete the code to use Kotlin's delegation syntax for the interface Printer.

Kotlin

interface Printer {
    fun print()
}

class RealPrinter : Printer {
    override fun print() {
        println("Real printing")
    }
}

class Document(printer: Printer) : Printer by [1] {
}

fun main() {
    val doc = Document(RealPrinter())
    doc.print()
}

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Aprinter

BRealPrinter()

Cthis

DDocument()

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'RealPrinter()' directly instead of the parameter name.

Using 'this' or 'Document()' which are incorrect here.

The class Document delegates the Printer interface implementation to the printer object passed in the constructor.

3fill in blank

hard

Fix the error by completing the code to delegate the interface correctly.

Kotlin

interface Logger {
    fun log(message: String)
}

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

class Service : Logger by [1] {
}

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

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Athis

BConsoleLogger

CConsoleLogger()

DLogger()

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name without parentheses causes an error.

Using 'this' or 'Logger()' which are invalid here.

The delegation requires an instance of ConsoleLogger, so ConsoleLogger() is correct.

4fill in blank

hard

Fill both blanks to create a class that delegates Logger and adds a new function.

Kotlin

interface Logger {
    fun log(message: String)
}

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

class AppLogger([1]: Logger) : Logger by [2] {
    fun error(message: String) {
        log("Error: $message")
    }
}

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Alogger

Blog

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the parameter and delegation causes errors.

Using 'log' which is a function name, not a variable.

The constructor parameter logger is used to delegate the Logger interface.

5fill in blank

hard

Fill all three blanks to implement delegation with an additional method in Kotlin.

Kotlin

interface Speaker {
    fun speak()
}

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

class Robot([1]: Speaker) : Speaker by [2] {
    fun compute() {
        println("Computing...")
    }
}

fun main() {
    val robot = Robot([3])
    robot.speak()
    robot.compute()
}

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Ahuman

CHuman()

Drobot

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for parameter and delegation.

Passing 'robot' instead of 'Human()' when creating Robot.

The constructor parameter human is delegated to implement Speaker, and an instance Human() is passed when creating Robot.