Preventing overrides with final in Swift

Introduction

Using final stops other parts of your code from changing a class or method. This helps keep your code safe and clear.

When you want to make sure a class cannot be changed by others.

When you want to stop a method from being changed in child classes.

When you want to protect important code from accidental changes.

When you want to improve performance by telling Swift the code won't change.

When you want to keep your design simple and clear.

Syntax

Swift

final class ClassName {
    // class code
}

class ParentClass {
    final func methodName() {
        // method code
    }
}

You put final before class to stop the class from being inherited.

You put final before a method to stop it from being overridden in child classes.

Examples

This class Animal cannot be inherited by any other class.

Swift

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

The method start() cannot be changed in Car because it is marked final in Vehicle.

Swift

class Vehicle {
    final func start() {
        print("Starting engine")
    }
}

class Car: Vehicle {
    // Trying to override start() here will cause an error
}

Sample Program

This program shows a Bird class with a final method fly(). The Eagle class tries to override it but cannot. When we call fly() on an Eagle object, it runs the original method.

Swift

class Bird {
    final func fly() {
        print("Flying high")
    }
}

class Eagle: Bird {
    // Uncommenting the below code will cause an error
    // override func fly() {
    //     print("Eagle flying")
    // }
}

let eagle = Eagle()
eagle.fly()

Important Notes

Marking a class final means no other class can inherit from it.

Marking a method final means child classes cannot change how it works.

Using final can help your app run faster because Swift knows the code won't change.

Summary

final stops classes or methods from being changed.

Use it to protect important code and keep your design clear.

It can also help your program run better.

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