Final classes and methods in PHP - Time & Space Complexity

Time Complexity: Final classes and methods

O(n)

Understanding Time Complexity

Let's see how using final classes and methods affects how long a PHP program takes to run.

We want to know if marking classes or methods as final changes the speed as the program grows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

final class Calculator {
    public function add(array $numbers): int {
        $sum = 0;
        foreach ($numbers as $num) {
            $sum += $num;
        }
        return $sum;
    }
}

$calc = new Calculator();
$result = $calc->add([1, 2, 3, 4, 5]);
    

This code defines a final class with a method that adds numbers in an array.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through the array to add numbers.
• How many times: Once for each number in the input array.

How Execution Grows With Input

As the number of numbers grows, the time to add them grows too.

Input Size (n)	Approx. Operations
10	About 10 additions
100	About 100 additions
1000	About 1000 additions

Pattern observation: The work grows directly with the number of items.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows in a straight line with the input size.

Common Mistake

[X] Wrong: "Making a class or method final makes the code run faster."

[OK] Correct: Final only stops changes in code structure; it does not change how many steps the program takes.

Interview Connect

Understanding time complexity helps you explain how your code behaves as it grows, which is a useful skill in many coding discussions.

Self-Check

"What if the add method used recursion instead of a loop? How would the time complexity change?"

Practice

1. What does the final keyword do when used with a class in PHP?

easy

A. Prevents the class from being extended by other classes

B. Allows the class to be extended multiple times

C. Makes the class abstract

D. Allows the class methods to be overridden

Solution

1. Step 1: Understand the role of final with classes

   The final keyword before a class means no other class can extend it.

2. Step 2: Compare with other options

   Options A, B, and D describe behaviors that are opposite or unrelated to final classes.

3. Final Answer:

   Prevents the class from being extended by other classes -> Option A

4. Quick Check:

   final class = no extension allowed [OK]

Hint: Final class means no child classes allowed [OK]

Common Mistakes:

• Thinking final class can be extended
• Confusing final with abstract
• Assuming final allows method overrides

2. Which of the following is the correct syntax to declare a final method in a PHP class?

easy

A. function final myMethod() {}

B. final function myMethod() {}

C. function myMethod() final {}

D. final myMethod function() {}

Solution

1. Step 1: Recall correct final method syntax

   In PHP, the final keyword comes before the function keyword when declaring a final method.

2. Step 2: Check each option

   final function myMethod() {} matches the correct syntax. Options A, C, and D have incorrect order or placement of keywords.

3. Final Answer:

   final function myMethod() {} -> Option B

4. Quick Check:

   final before function keyword = correct syntax [OK]

Hint: final keyword always before function name in declaration [OK]

Common Mistakes:

• Placing final after function name
• Writing final after function keyword incorrectly
• Mixing order of keywords

3. What will be the output of the following PHP code?

final class Car {
    final public function start() {
        return "Engine started";
    }
}

class SportsCar extends Car {
    public function start() {
        return "Sports engine started";
    }
}

$car = new SportsCar();
echo $car->start();

medium

A. Engine started

B. Sports engine started

C. Fatal error: Cannot override final method Car::start()

D. Fatal error: Cannot extend final class Car

Solution

1. Step 1: Analyze class and method declarations

   The class Car is declared as final, so it cannot be extended. But the code tries to extend it with SportsCar.

2. Step 2: Identify the error

   Since Car is final, extending it causes a fatal error. But the question is about overriding a final method. Actually, the class is final, so extending is not allowed at all.

3. Step 3: Check options carefully

   Fatal error: Cannot extend final class Car says cannot extend final class, which matches the error. Fatal error: Cannot override final method Car::start() says cannot override final method, which would be error if class was not final but method was final.

4. Final Answer:

   Fatal error: Cannot extend final class Car -> Option D

5. Quick Check:

   final class cannot be extended = fatal error [OK]

Hint: Final class cannot be extended, causes fatal error [OK]

Common Mistakes:

• Confusing final method error with final class error
• Assuming method override works despite final class
• Ignoring that class is final and cannot be extended

4. Identify the error in the following PHP code:

class Vehicle {
    final public function move() {
        return "Moving";
    }
}

class Bike extends Vehicle {
    public function move() {
        return "Bike moving";
    }
}

medium

A. No error, code runs fine

B. Cannot extend class Vehicle

C. Cannot override final method move() in class Bike

D. Syntax error in method declaration

Solution

1. Step 1: Understand final method behavior

   The method move() in Vehicle is declared as final, so child classes cannot override it.

2. Step 2: Check the child class method

   The Bike class tries to override move(), which is not allowed and causes a fatal error.

3. Final Answer:

   Cannot override final method move() in class Bike -> Option C

4. Quick Check:

   final method cannot be overridden [OK]

Hint: Final method cannot be changed in child classes [OK]

Common Mistakes:

• Thinking final method can be overridden
• Confusing final method with abstract method
• Assuming no error when overriding final method

5. You want to create a base class Document that cannot be extended, but you want to allow extending a method print() in child classes. Which is the correct approach?

hard

A. Do not declare class Document as final, and declare method print() normally

B. Declare class Document as final, and method print() normally (not final)

C. Do not declare class Document as final, but declare method print() as final

D. Declare class Document as final, and method print() as final

Solution

1. Step 1: Understand final class effect

   Declaring a class as final prevents any extension, so no child classes can exist to override methods.

2. Step 2: Understand method overriding

   To allow overriding print(), the class must be extendable and the method must not be final.

3. Step 3: Choose correct option

   Do not declare class Document as final, and declare method print() normally allows extending the class and overriding the method. Options B and D prevent extension. Do not declare class Document as final, but declare method print() as final prevents method overriding.

4. Final Answer:

   Do not declare class Document as final, and declare method print() normally -> Option A

5. Quick Check:

   Allow extension and method override = no final on class or method [OK]

Hint: Final class blocks extension; omit final to allow overrides [OK]

Common Mistakes:

• Declaring class final but expecting method override
• Declaring method final when override is needed
• Confusing final class and final method effects