Interface declaration and implementation in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of interface method call

What is the output of this Kotlin code when calling greet() on an instance of FriendlyPerson?

Kotlin

interface Greeter {
    fun greet(): String
}

class FriendlyPerson : Greeter {
    override fun greet(): String {
        return "Hello, friend!"
    }
}

fun main() {
    val person: Greeter = FriendlyPerson()
    println(person.greet())
}

A Hello, world!

B Compilation error: greet() not implemented

C Hello, friend!

D Runtime error: NullPointerException

💡 Hint

Check how the greet() method is implemented in the class.

The class FriendlyPerson implements the Greeter interface and overrides the greet() method to return "Hello, friend!". So calling greet() prints that string.

🧠 Conceptual

intermediate

Interface property declaration

Which of the following is a valid way to declare a read-only property in a Kotlin interface?

A interface Vehicle { var wheels: Int = 4 }

B interface Vehicle { val wheels: Int }

C interface Vehicle { fun wheels(): Int = 4 }

D interface Vehicle { val wheels: Int = 4 }

💡 Hint

Interfaces can declare properties without initial values.

In Kotlin, interfaces can declare abstract properties without initial values. Option B declares a read-only property wheels without initializing it, which is valid. Option B tries to initialize a var property, which is not allowed. Option B declares a function, not a property. Option B tries to initialize a val property, which is not allowed in interfaces.

🔧 Debug

advanced

Identify the error in interface implementation

What error will this Kotlin code produce?

Kotlin

interface Speaker {
    fun speak()
}

class Person : Speaker {
    fun speak() {
        println("Hi!")
    }
}

fun main() {
    val p = Person()
    p.speak()
}

A Compilation error: 'speak' in 'Person' hides member of supertype 'Speaker' and needs 'override' modifier

B Runtime error: Method not found

C No error, prints 'Hi!'

D Compilation error: Interface cannot have methods without body

💡 Hint

Check if the method correctly overrides the interface method.

In Kotlin, when a class implements an interface method, it must use the 'override' keyword. The code misses 'override' before 'fun speak()', causing a compilation error.

❓ Predict Output

advanced

Multiple interface inheritance output

What is the output of this Kotlin code?

Kotlin

interface A {
    fun hello() = "Hello from A"
}

interface B {
    fun hello() = "Hello from B"
}

class C : A, B {
    override fun hello(): String {
        return super<A>.hello() + " & " + super<B>.hello()
    }
}

fun main() {
    val c = C()
    println(c.hello())
}

A Hello from A & Hello from B

B Hello from B & Hello from A

C Runtime error: StackOverflowError

D Compilation error: Ambiguous call to hello()

💡 Hint

Look at how the class C calls the super implementations explicitly.

Class C overrides hello() and calls both interface implementations explicitly using super and super. So it prints "Hello from A & Hello from B".

🧠 Conceptual

expert

Interface delegation behavior

Given the following Kotlin code, what will be printed when main() runs?

Kotlin

interface Logger {
    fun log(message: String)
}

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

class Application(logger: Logger) : Logger by logger {
    fun run() {
        log("Application started")
    }
}

fun main() {
    val logger = ConsoleLogger()
    val app = Application(logger)
    app.run()
}

A Application started

B Runtime error: log() method not found

C Compilation error: Delegation syntax incorrect

D Console: Application started

💡 Hint

Interface delegation forwards calls to the provided object.

The Application class delegates Logger interface to the passed logger instance. So calling log() inside Application calls ConsoleLogger's log(), printing "Console: Application started".