Interface with default implementations in Kotlin - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of interface default method call

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

Kotlin

interface Greeter {
    fun greet() {
        println("Hello from Greeter")
    }
}

class Person : Greeter

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

A Hello from Greeter

B Compilation error: greet() must be overridden

C Runtime error: greet() not implemented

D No output

💡 Hint

Interfaces in Kotlin can have default method implementations.

Since Greeter interface provides a default implementation of greet(), the Person class inherits it without needing to override. So calling greet() prints the default message.

❓ Predict Output

intermediate

Output when overriding default method

What will be printed when greet() is called on Student instance?

Kotlin

interface Greeter {
    fun greet() {
        println("Hello from Greeter")
    }
}

class Student : Greeter {
    override fun greet() {
        println("Hello from Student")
    }
}

fun main() {
    val s = Student()
    s.greet()
}

A Hello from Student

B Hello from Greeter

C Compilation error: greet() cannot override default

D Runtime error: greet() not found

💡 Hint

Overriding a default method replaces the original implementation.

The Student class overrides the default greet() method, so calling it prints the overridden message.

🔧 Debug

advanced

Identify the error in interface default method usage

What error occurs when compiling this Kotlin code?

Kotlin

interface A {
    fun show() {
        println("A show")
    }
}

interface B {
    fun show() {
        println("B show")
    }
}

class C : A, B {
    override fun show() {
        super.show()
    }
}

fun main() {
    val c = C()
    c.show()
}

A No error, prints 'B show'

B Runtime error: super.show() not found

C No error, prints 'A show'

D Compilation error: 'super' call is ambiguous between A and B

💡 Hint

When multiple interfaces have the same default method, you must specify which one to call.

Class C inherits two interfaces with the same default method. Calling super.show() is ambiguous and causes a compilation error. You must specify which interface's method to call using syntax like super.show() or super.show().

📝 Syntax

advanced

Correct syntax to call specific interface default method

Which option correctly calls the show() method from interface B inside class C?

Kotlin

interface A {
    fun show() {
        println("A show")
    }
}

interface B {
    fun show() {
        println("B show")
    }
}

class C : A, B {
    override fun show() {
        // call B's show here
    }
}

A super.show()

B super::B.show()

C super<B>.show()

D B.show()

💡 Hint

Use the syntax super.methodName() to call a specific interface's default method.

To resolve ambiguity, Kotlin requires specifying the interface name in super call like super.show().

🚀 Application

expert

Determine output with multiple interface default methods and override

What is the output of this Kotlin program?

Kotlin

interface X {
    fun action() {
        println("X action")
    }
}

interface Y {
    fun action() {
        println("Y action")
    }
}

class Z : X, Y {
    override fun action() {
        super<X>.action()
        super<Y>.action()
        println("Z action")
    }
}

fun main() {
    val z = Z()
    z.action()
}

A X action\nZ action

B X action\nY action\nZ action

C Compilation error: ambiguous super calls

D Y action\nZ action

💡 Hint

You can call multiple interface default methods explicitly using super.method()

Class Z overrides action() and calls both interface default implementations explicitly, then prints its own message. So all three lines print in order.