Open classes and methods in Kotlin - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the open keyword mean in Kotlin?

In Kotlin, <code>open</code> means a class or method can be inherited or overridden. By default, classes and methods are <code>final</code> and cannot be changed.

beginner

Why are classes and methods final by default in Kotlin?

Kotlin makes classes and methods final by default to avoid accidental inheritance or overriding, which helps keep code safer and easier to understand.

beginner

How do you make a class inheritable in Kotlin?

Add the <code>open</code> keyword before the class name. For example: <code>open class Animal</code> allows other classes to inherit from <code>Animal</code>.

beginner

How do you allow a method to be overridden in Kotlin?

Mark the method with open. For example: open fun speak() {} lets subclasses provide their own version of speak().

beginner

What happens if you try to override a method that is not marked open?

Kotlin will give a compile error because only open methods can be overridden. This prevents accidental changes to important behavior.

In Kotlin, what keyword do you use to allow a class to be inherited?

A open

B final

C override

D abstract

The open keyword allows a class to be inherited. Classes are final by default.

What is the default behavior of classes in Kotlin regarding inheritance?

A They are abstract

B They are open for inheritance

C They are final and cannot be inherited

D They are interfaces

By default, Kotlin classes are final and cannot be inherited unless marked open.

Which keyword allows a method to be overridden in Kotlin?

A abstract

B override

C final

D open

The open keyword marks a method as overridable. override is used when overriding.

What happens if you try to override a method not marked as open?

A It works normally

B You get a compile-time error

C The method becomes abstract

D The method is ignored

Kotlin prevents overriding methods that are not open by giving a compile-time error.

Which of these is true about open classes and methods?

A They can be inherited or overridden

B They cannot be inherited or overridden

C They are abstract by default

D They are deprecated

open means classes or methods can be inherited or overridden.

Explain what the open keyword does in Kotlin and why it is important.

Think about how Kotlin protects your code by default and how <code>open</code> changes that.

Describe the difference between a class marked open and one that is not in Kotlin.

Focus on inheritance and overriding permissions.

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