Open classes and methods in Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to make the class extendable.

Kotlin

open class Vehicle {
    fun drive() {
        println("Driving")
    }
}

class Car : [1] Vehicle() {}

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

Aabstract

Bopen

Cfinal

Dsealed

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' which prevents inheritance.

Forgetting to mark the class as 'open'.

In Kotlin, to allow a class to be inherited, it must be declared as open.

2fill in blank

medium

Complete the code to allow the method to be overridden.

Kotlin

open class Animal {
    [1] fun sound() {
        println("Some sound")
    }
}

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

Aopen

Bfinal

Coverride

Dabstract

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'final' which prevents overriding.

Using 'override' keyword in the base class.

Methods must be marked open to be overridden in subclasses.

3fill in blank

hard

Fix the error in the subclass method overriding.

Kotlin

open class Bird {
    open fun fly() {
        println("Flying")
    }
}

class Eagle : Bird() {
    override fun [1]() {
        println("Eagle flying")
    }
}

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

Aswim

Brun

Cfly

Djump

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different method name in the override.

Forgetting to mark the base method as open.

The method name in the subclass must match the open method in the superclass to override it.

4fill in blank

hard

Fill both blanks to create an open class with an open method.

Kotlin

[1] class Shape {
    [2] fun draw() {
        println("Drawing shape")
    }
}

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

Aopen

Bfinal

Dabstract

Attempts:

3 left

💡 Hint

Common Mistakes

Marking class as open but method as final.

Forgetting to mark either class or method as open.

Both the class and the method must be marked open to allow inheritance and overriding.

5fill in blank

hard

Fill all three blanks to override an open method in a subclass.

Kotlin

open class Printer {
    open fun [1]() {
        println("Printing document")
    }
}

class ColorPrinter : Printer() {
    override fun [2]() {
        println([3])
    }
}

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

Aprint

C"Printing in color"

D"Printing in black and white"

Attempts:

3 left

💡 Hint

Common Mistakes

Using different method names in override.

Printing wrong message in subclass.

The method name must match in both classes, and the subclass method can provide a different implementation.

Practice

1. In Kotlin, what does the open keyword do when applied to a class or method?

easy

A. Prevents the class or method from being used

B. Makes the class or method private

C. Allows the class or method to be inherited or overridden

D. Automatically overrides the method in subclasses

Solution

1. Step 1: Understand Kotlin class and method defaults

   By default, Kotlin classes and methods are final, meaning they cannot be inherited or overridden.

2. Step 2: Role of the open keyword

   The open keyword explicitly allows a class to be inherited or a method to be overridden in subclasses.

3. Final Answer:

   Allows the class or method to be inherited or overridden -> Option C

4. Quick Check:

   open means inheritance allowed [OK]

Hint: Remember: open means 'can be changed' [OK]

Common Mistakes:

• Thinking classes are open by default
• Confusing open with override
• Assuming open makes methods private

2. Which of the following is the correct way to declare an open method named display in Kotlin?

easy

A. open fun display() { }

B. fun display() open { }

C. fun open display() { }

D. open display fun() { }

Solution

1. Step 1: Recall Kotlin method declaration syntax

   In Kotlin, the keyword fun starts the function declaration, followed by the function name and parentheses.

2. Step 2: Position of open keyword

   The open keyword must come before fun to mark the method as open for overriding.

3. Final Answer:

   open fun display() { } -> Option A

4. Quick Check:

   open fun is correct order [OK]

Hint: Put open before fun for methods [OK]

Common Mistakes:

• Placing open after fun
• Putting open after method name
• Using open without fun keyword

3. What will be the output of the following Kotlin code?

open class Animal {
    open fun sound() = "Some sound"
}

class Dog : Animal() {
    override fun sound() = "Bark"
}

fun main() {
    val pet: Animal = Dog()
    println(pet.sound())
}

medium

A. Some sound

B. Bark

C. Compilation error

D. Runtime error

Solution

1. Step 1: Understand class inheritance and method overriding

   The class Dog inherits from Animal and overrides the sound() method to return "Bark".

2. Step 2: Polymorphism in action

   Even though pet is typed as Animal, it holds a Dog instance, so the overridden sound() method in Dog is called.

3. Final Answer:

   Bark -> Option B

4. Quick Check:

   Overridden method runs on subclass instance [OK]

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

Common Mistakes:

• Expecting base class method output
• Confusing compile-time and run-time types
• Forgetting to mark methods open

4. Identify the error in the following Kotlin code:

class Vehicle {
    open fun start() = "Starting"
}

class Car : Vehicle() {
    fun start() = "Car starting"
}

medium

A. Method start() in Car must be marked override

B. No error, code is correct

C. Car class cannot inherit from Vehicle

D. Vehicle class must be open to allow inheritance

Solution

1. Step 1: Check if Vehicle class allows inheritance

   By default, Kotlin classes are final. Since Vehicle is not marked open, it cannot be inherited.

2. Step 2: Analyze the inheritance attempt

   Car tries to inherit from Vehicle, which causes a compilation error because Vehicle is not open.

3. Final Answer:

   Vehicle class must be open to allow inheritance -> Option D

4. Quick Check:

   Classes must be open to inherit [OK]

Hint: Mark classes open to inherit them [OK]

Common Mistakes:

• Forgetting to mark base class open
• Assuming methods override without keyword
• Ignoring Kotlin's default final classes

5. You want to create a Kotlin class Shape with an open method area() that returns 0.0 by default. Then create a subclass Circle that overrides area() to return the circle's area using its radius. Which code correctly implements this?

hard

A. open class Shape {
open fun area() = 0.0
}

class Circle(val radius: Double) : Shape() {
override fun area() = 3.14 * radius * radius
}

B. class Shape {
fun area() = 0.0
}

class Circle(val radius: Double) : Shape() {
fun area() = 3.14 * radius * radius
}

C. open class Shape {
fun area() = 0.0
}

class Circle(val radius: Double) : Shape() {
override fun area() = 3.14 * radius * radius
}

D. class Shape {
open fun area() = 0.0
}

class Circle(val radius: Double) : Shape() {
override fun area() = 3.14 * radius * radius
}

Solution

1. Step 1: Mark base class and method open

   To allow inheritance and overriding, Shape must be open and its area() method must also be open.

2. Step 2: Override method correctly in subclass

   Circle inherits from Shape and overrides area() using override keyword, calculating area using radius.

3. Final Answer:

   Option A code correctly implements open class and method with override -> Option A

4. Quick Check:

   Open base and override subclass method [OK]

Hint: Open base class and method, override in subclass [OK]

Common Mistakes:

• Not marking class or method open
• Missing override keyword in subclass
• Trying to override non-open method