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.