Preventing overrides with final in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to prevent the method from being overridden.

Swift

class Vehicle {
    [1] func startEngine() {
        print("Engine started")
    }
}

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

Aopen

Boverride

Cfinal

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'override' instead of 'final'.

Using 'open' which allows overriding.

Forgetting to add any keyword.

Using 'final' before a method prevents subclasses from overriding it.

2fill in blank

medium

Complete the code to prevent the class from being subclassed.

Swift

[1] class Animal {
    func sound() {
        print("Some sound")
    }
}

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

Afinal

Bopen

Cpublic

Doverride

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'open' which allows subclassing.

Using 'override' which is for methods, not classes.

Not adding any keyword.

Marking a class as 'final' stops other classes from inheriting it.

3fill in blank

hard

Fix the error in the subclass trying to override a final method.

Swift

class Car {
    final func drive() {
        print("Driving")
    }
}

class SportsCar: Car {
    [1] func drive() {
        print("Fast driving")
    }
}

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

Aopen

Boverride

Cpublic

Dfinal

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'override' keyword on a final method.

Trying to change access level with 'public' or 'open'.

You cannot override a final method; removing 'override' or marking method as 'final' in subclass is needed.

4fill in blank

hard

Fill both blanks to prevent subclassing and method overriding.

Swift

[1] class Bird {
    [2] func fly() {
        print("Flying")
    }
}

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

Afinal

Bopen

Cpublic

Doverride

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'open' which allows subclassing and overriding.

Mixing 'public' and 'override' keywords incorrectly.

Marking both the class and method as 'final' stops subclassing and overriding.

5fill in blank

hard

Fill all three blanks to create a final class with a final method and a non-final method.

Swift

[1] class Plane {
    [2] func takeOff() {
        print("Taking off")
    }
    [3] func land() {
        print("Landing")
    }
}

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

Afinal

Bopen

Coverride

Dpublic

Attempts:

3 left

💡 Hint

Common Mistakes

Marking the class as 'open' which allows subclassing.

Using 'override' keyword incorrectly on methods.

Not marking the method as 'final' to prevent overriding.

The class and takeOff method are final to prevent subclassing and overriding; land method is public and can be overridden if subclassing was allowed.

Practice

1.

What does the final keyword do in Swift?

easy

A. Makes a method asynchronous

B. Allows a class to be subclassed multiple times

C. Marks a variable as constant

D. Prevents a class or method from being overridden or subclassed

Solution

1. Step 1: Understand the purpose of final

   The final keyword in Swift is used to stop classes or methods from being changed by subclasses.

2. Step 2: Identify what final prevents

   It prevents overriding methods or subclassing classes, ensuring the code stays as originally designed.

3. Final Answer:

   Prevents a class or method from being overridden or subclassed -> Option D

4. Quick Check:

   final stops overrides = A [OK]

Hint: Remember: final means no changes allowed to class or method [OK]

Common Mistakes:

• Confusing final with constant variables
• Thinking final allows subclassing
• Mixing final with async keyword

2.

Which of the following is the correct way to declare a method that cannot be overridden in Swift?

class Vehicle {
    _____ func startEngine() {
        print("Engine started")
    }
}

easy

A. override

B. mutating

C. final

D. static

Solution

1. Step 1: Identify the keyword to prevent overriding

   In Swift, the final keyword is used before a method to stop it from being overridden in subclasses.

2. Step 2: Check the method declaration

   Placing final before func correctly marks the method as non-overridable.

3. Final Answer:

   final -> Option C

4. Quick Check:

   Use final before func to prevent override [OK]

Hint: final before func means no override allowed [OK]

Common Mistakes:

• Using override keyword incorrectly
• Confusing static with final
• Using mutating which is for structs

3.

What will happen when you run this Swift code?

class Animal {
    final func sound() {
        print("Animal sound")
    }
}

class Dog: Animal {
    override func sound() {
        print("Bark")
    }
}

let pet = Dog()
pet.sound()

medium

A. Prints "Animal sound"

B. Compile-time error due to overriding a final method

C. Prints "Bark"

D. Runtime error

Solution

1. Step 1: Identify the use of final in the base class

   The method sound() in Animal is marked as final, so it cannot be overridden.

2. Step 2: Check the subclass method override

   The Dog class tries to override sound(), which causes a compile-time error because overriding a final method is not allowed.

3. Final Answer:

   Compile-time error due to overriding a final method -> Option B

4. Quick Check:

   Overriding final method = Compile error [OK]

Hint: Overriding final method causes compile error [OK]

Common Mistakes:

• Assuming it prints "Bark"
• Thinking it prints base method output
• Expecting runtime error instead of compile error

4.

Find the error in this Swift code and fix it:

final class Vehicle {
    func drive() {
        print("Driving")
    }
}

class Car: Vehicle {
    override func drive() {
        print("Car driving")
    }
}

medium

A. Remove final from Vehicle class

B. Remove override from Car's drive method

C. Change Vehicle to a struct

D. Add final to Car class

Solution

1. Step 1: Identify the use of final on the Vehicle class

   The Vehicle class is marked as final, so it cannot be subclassed.

2. Step 2: Check subclass declaration

   The Car class tries to subclass Vehicle, which causes a compile-time error because subclassing a final class is not allowed.

3. Final Answer:

   Remove final from Vehicle class -> Option A

4. Quick Check:

   Subclassing final class = Error; remove final [OK]

Hint: final class cannot be subclassed; remove final to fix [OK]

Common Mistakes:

• Trying to override without removing final
• Adding final to subclass instead of base
• Changing class to struct unnecessarily

5.

You want to create a base class BankAccount with a method calculateInterest() that should never be changed by subclasses. However, subclasses can add new methods. How should you declare BankAccount and calculateInterest()?

hard

A. Declare BankAccount normally and calculateInterest() as final

B. Declare both BankAccount and calculateInterest() as final

C. Declare BankAccount as final and calculateInterest() normally

D. Declare BankAccount as a struct and calculateInterest() as final

Solution

1. Step 1: Understand the requirement for subclassing

   The base class BankAccount should allow subclasses, so it must NOT be final.

2. Step 2: Protect the method from being overridden

   Mark calculateInterest() as final to prevent subclasses from changing it.

3. Final Answer:

   Declare BankAccount normally and calculateInterest() as final -> Option A

4. Quick Check:

   final method, normal class = C [OK]

Hint: final method stops override; class normal allows subclass [OK]

Common Mistakes:

• Making the whole class final, blocking subclassing
• Not marking method as final, allowing override
• Using struct instead of class incorrectly