Interface with default implementations in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an interface with a default implementation for the greet function.

Kotlin

interface Greeter {
    fun greet() {
        println([1])
    }
}

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

A"Hello from interface!"

Bgreet()

Cprintln("Hello")

Dfun greet()

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to call greet() inside its own body causing recursion.

Using function declaration syntax inside the function body.

The greet function in the interface has a default implementation that prints the greeting message.

2fill in blank

medium

Complete the code to implement the Greeter interface and override the greet function.

Kotlin

class FriendlyGreeter : Greeter {
    override fun greet() {
        println([1])
    }
}

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

Agreet()

B"Hi there!"

Cprintln("Hello")

Dsuper.greet()

Attempts:

3 left

💡 Hint

Common Mistakes

Calling greet() inside itself causing infinite recursion.

Trying to call super.greet() without a proper context.

The FriendlyGreeter class overrides greet to print a custom message.

3fill in blank

hard

Fix the error in the code to call the default greet implementation from the interface inside the override.

Kotlin

class PoliteGreeter : Greeter {
    override fun greet() {
        [1].greet()
        println("Have a nice day!")
    }
}

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

Athis

Bsuper

CGreeter.super

DPoliteGreeter.super

Attempts:

3 left

💡 Hint

Common Mistakes

Using just super.greet() which is invalid for interfaces.

Using this.greet() causing recursion.

To call the interface's default implementation, use InterfaceName.super.methodName() syntax.

4fill in blank

hard

Fill both blanks to define an interface with a default method and a class that calls it.

Kotlin

interface Speaker {
    fun speak() {
        println([1])
    }
}

class Person : Speaker {
    fun sayHello() {
        [2].speak()
    }
}

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

A"Hello!"

BSpeaker.super

Cthis

D"Hi!"

Attempts:

3 left

💡 Hint

Common Mistakes

Using this.speak() instead of Speaker.super.speak().

Not providing a string literal in the interface method.

The interface method prints "Hello!" and the class calls the default speak using Speaker.super.

5fill in blank

hard

Fill all three blanks to create an interface with a default method, a class overriding it, and calling both versions.

Kotlin

interface Animal {
    fun sound() {
        println([1])
    }
}

class Dog : Animal {
    override fun sound() {
        println([2])
        [3].sound()
    }
}

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

A"Generic animal sound"

B"Bark!"

CAnimal.super

D"Meow!"

Attempts:

3 left

💡 Hint

Common Mistakes

Using super.sound() instead of Animal.super.sound() for interface default call.

Not overriding the sound method properly.

The interface prints a generic sound, Dog overrides with "Bark!" and calls the default using Animal.super.sound().

Practice

1.

What is the main purpose of providing a default implementation in a Kotlin interface?

interface Example { fun greet() { println("Hello") } }

easy

A. To allow classes to use the function without implementing it themselves

B. To prevent classes from overriding the function

C. To make the interface abstract and unusable

D. To force all classes to implement the function

Solution

1. Step 1: Understand default implementations in interfaces

   Default implementations provide a function body inside the interface itself.

2. Step 2: Effect on implementing classes

   Classes can use this default function without writing their own version.

3. Final Answer:

   To allow classes to use the function without implementing it themselves -> Option A

4. Quick Check:

   Default implementation means no need to override [OK]

Hint: Default means usable without override [OK]

Common Mistakes:

• Thinking default means function cannot be overridden
• Confusing default implementation with abstract function
• Believing interface becomes unusable with default methods

2.

Which of the following is the correct syntax for a Kotlin interface with a default implementation of a function fun info()?

easy

A. interface Info { fun info() = println("Info") }

B. interface Info { fun info() println("Info") }

C. interface Info { fun info() -> println("Info") }

D. interface Info { fun info; println("Info") }

Solution

1. Step 1: Recall Kotlin interface function syntax

   Default implementations use = followed by expression or block with braces.

2. Step 2: Check each option

   interface Info { fun info() = println("Info") } uses = with println(), which is correct syntax for default implementation.

3. Final Answer:

   interface Info { fun info() = println("Info") } -> Option A

4. Quick Check:

   Default function uses = expression [OK]

Hint: Default functions use = for single expression [OK]

Common Mistakes:

• Using braces without = in interface default functions
• Using arrow -> instead of =
• Writing function body outside interface

3.

What will be the output of the following Kotlin code?

interface Printer {
    fun print() { println("Printing from interface") }
}

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

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

medium

A. Printing from interface

B. Compilation error due to override

C. Printing from Document

D. Runtime error

Solution

1. Step 1: Understand override behavior

   The class Document overrides the print() function from Printer interface.

2. Step 2: Check which print() is called

   Even though the variable type is Printer, the actual object is Document, so overridden print() runs.

3. Final Answer:

   Printing from Document -> Option C

4. Quick Check:

   Overridden method runs on object type [OK]

Hint: Overridden methods run from actual object type [OK]

Common Mistakes:

• Thinking interface default method runs instead of override
• Confusing variable type with object type
• Expecting compilation error for override

4.

Identify the error in this Kotlin code snippet:

interface Logger {
    fun log() { println("Logging") }
}

class FileLogger : Logger {
    fun log() { println("File logging") }
}

medium

A. FileLogger cannot implement Logger without constructor

B. Missing override keyword in FileLogger's log() function

C. Interface functions cannot have bodies

D. log() function must be abstract in interface

Solution

1. Step 1: Check interface function declaration

   Logger interface has a default implementation of log().

2. Step 2: Check FileLogger's log() function

   It defines log() but misses the override keyword, which is required.

3. Final Answer:

   Missing override keyword in FileLogger's log() function -> Option B

4. Quick Check:

   Override keyword required when overriding [OK]

Hint: Override keyword is mandatory for overrides [OK]

Common Mistakes:

• Forgetting override keyword
• Thinking interface functions cannot have bodies
• Assuming abstract keyword needed in interface

5.

Given this Kotlin interface and classes, what will be the output when main() runs?

interface Speaker {
    fun speak() { println("Speaking from Speaker") }
}

class Person : Speaker

class Robot : Speaker {
    override fun speak() { println("Speaking from Robot") }
}

fun main() {
    val list: List<Speaker> = listOf(Person(), Robot())
    for (item in list) {
        item.speak()
    }
}

hard

A. Compilation error due to missing override in Person

B. Speaking from Person
Speaking from Robot

C. Speaking from Speaker
Speaking from Speaker

D. Speaking from Speaker
Speaking from Robot

Solution

1. Step 1: Analyze Person class

   Person does not override speak(), so it uses Speaker's default implementation.

2. Step 2: Analyze Robot class

   Robot overrides speak(), so its own version runs.

3. Step 3: Loop output

   First item (Person) prints "Speaking from Speaker", second (Robot) prints "Speaking from Robot".

4. Final Answer:

   Speaking from Speaker Speaking from Robot -> Option D

5. Quick Check:

   Default + override methods run accordingly [OK]

Hint: Classes use default unless overridden [OK]

Common Mistakes:

• Expecting Person to print "Speaking from Person"
• Thinking missing override causes error
• Assuming both print Robot's message