Preventing overrides with final in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of overriding a final method

What will be the output of this Swift code?

Swift

class Vehicle {
    final func start() {
        print("Vehicle started")
    }
}

class Car: Vehicle {
    override func start() {
        print("Car started")
    }
}

let myCar = Car()
myCar.start()

A Runtime error

B Car started

C Vehicle started

D Compile-time error: Cannot override 'final' method

💡 Hint

Remember what the 'final' keyword means for methods in Swift.

The 'final' keyword prevents a method from being overridden in subclasses. Trying to override a final method causes a compile-time error.

❓ Predict Output

intermediate

Output when subclass tries to override a non-final method

What will be printed when running this Swift code?

Swift

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

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

let pet: Animal = Dog()
pet.sound()

A Bark

B Some sound

C Compile-time error: Cannot override method

D Runtime error

💡 Hint

Check if the method is marked final or not.

Since 'sound()' is not final, the Dog class can override it. The call uses dynamic dispatch and prints 'Bark'.

🔧 Debug

advanced

Identify the error caused by final class inheritance

What error will this Swift code produce?

Swift

final class Calculator {
    func add(_ a: Int, _ b: Int) -> Int {
        return a + b
    }
}

class AdvancedCalculator: Calculator {
    func multiply(_ a: Int, _ b: Int) -> Int {
        return a * b
    }
}

A Runtime error: Inheritance violation

B Compile-time error: Cannot inherit from final class 'Calculator'

C No error, code runs fine

D Compile-time error: Method multiply missing override keyword

💡 Hint

Check the meaning of 'final' when applied to classes.

A class marked as 'final' cannot be subclassed. Trying to inherit from it causes a compile-time error.

📝 Syntax

advanced

Which code correctly prevents overriding a method?

Choose the Swift code snippet that correctly prevents subclasses from overriding the method 'display()'.

A

class Screen {
    final func display() {
        print("Display on screen")
    }
}

B

final func display() {
    print("Display on screen")
}

C

class Screen {
    func final display() {
        print("Display on screen")
    }
}

D

class Screen {
    func display() final {
        print("Display on screen")
    }
}

💡 Hint

The 'final' keyword must come before 'func' in method declaration inside a class.

Option A uses the correct syntax: 'final func' inside a class. Other options have syntax errors or misplaced keywords.

🚀 Application

expert

How many methods can be overridden?

Given the following Swift code, how many methods can subclasses override?

Swift

class Base {
    final func fixedMethod() {}
    func openMethod() {}
    private func privateMethod() {}
    fileprivate func fileMethod() {}
}

A 1

B 3

C 2

D 0

💡 Hint

Consider method access levels and the effect of 'final'.

Only 'openMethod()' and 'fileMethod()' can be overridden by subclasses in the same file. 'fixedMethod()' is final and cannot be overridden. 'privateMethod()' is not visible to subclasses.

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