Preventing overrides with final in Swift - Mini Project: Build & Apply

Preventing Overrides with final in Swift

📖 Scenario: Imagine you are creating a simple app to manage vehicles. You want to make sure that certain behaviors of your vehicle classes cannot be changed by mistake in subclasses.

🎯 Goal: You will create a base class with a method that cannot be overridden by subclasses using the final keyword. This helps keep your code safe and predictable.

📋 What You'll Learn

Create a class called Vehicle with a method startEngine() that prints a message.

Add the final keyword to the startEngine() method to prevent overriding.

Create a subclass called Car that inherits from Vehicle.

Try to override the startEngine() method in Car (this should cause an error if uncommented).

Create an instance of Car and call the startEngine() method.

Print the output to see the behavior.

💡 Why This Matters

🌍 Real World

In real apps, you often want to protect core behaviors of your classes so they don't change unexpectedly when others extend your code.

💼 Career

Understanding <code>final</code> helps you write safer, more reliable Swift code, a skill valued in iOS and macOS development jobs.

1

Create the Vehicle class with a startEngine() method

Create a class called Vehicle with a method startEngine() that prints "Engine started".

Swift

# Create the Vehicle class with startEngine() method
# Your code here

Hint

Use class Vehicle {} and define func startEngine() inside it.

2

Make startEngine() method final to prevent overrides

Add the final keyword before the func startEngine() method in the Vehicle class to prevent it from being overridden.

Swift

class Vehicle {
    // Make startEngine() final
    # Your code here
}

Hint

Write final func startEngine() to stop subclasses from changing this method.

3

Create a Car subclass inheriting from Vehicle

Create a subclass called Car that inherits from Vehicle. Inside Car, add a commented-out attempt to override startEngine() method that prints "Car engine started".

Swift

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

// Create Car subclass here
# Your code here

Hint

Use class Car: Vehicle {} and inside it write a commented override of startEngine().

4

Create a Car instance and call startEngine()

Create an instance called myCar of the Car class and call the startEngine() method on it. Print the output.

Swift

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

class Car: Vehicle {
    // This override is not allowed because startEngine() is final
    // override func startEngine() {
    //     print("Car engine started")
    // }
}

// Create myCar instance and call startEngine()
# Your code here

Hint

Write let myCar = Car() and then myCar.startEngine().

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