Preventing overrides with final in Swift - Time & Space Complexity

Time Complexity: Preventing overrides with final

O(n)

Understanding Time Complexity

Let's see how using final affects the time it takes for Swift to run code involving class methods.

We want to know if marking methods as final changes how the program's speed grows with bigger inputs.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

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

class Dog: Animal {
    // Cannot override speak() because it's final
}

let dog = Dog()
dog.speak()
    

This code defines a method that cannot be changed by subclasses, then calls it.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Calling the speak() method once.
• How many times: Exactly one time in this example.

How Execution Grows With Input

Since the method is called once, the time to run does not grow with input size.

Input Size (n)	Approx. Operations
10	10 calls, each direct and fixed time
100	100 calls, each direct and fixed time
1000	1000 calls, each direct and fixed time

Pattern observation: The time grows directly with the number of calls, but each call is simple and fast because final avoids extra checks.

Final Time Complexity

Time Complexity: O(n)

This means the total time grows in a straight line with how many times you call the method, and each call is quick because it is fixed and direct.

Common Mistake

[X] Wrong: "Using final makes the method run instantly no matter how many times it's called."

[OK] Correct: Even with final, each call takes some time, so if you call it many times, total time still grows.

Interview Connect

Understanding how final affects method calls helps you explain how Swift optimizes code and why some choices make programs faster.

Self-Check

What if the speak() method was not marked final and was overridden in subclasses? How would the time complexity change?

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