Factory pattern in PHP - Time & Space Complexity

Time Complexity: Factory pattern

O(n)

Understanding Time Complexity

We want to understand how the time it takes to create objects using the factory pattern changes as we create more objects.

How does the work grow when we ask the factory to make many objects?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class Car {
    public function drive() {
        echo "Driving a car\n";
    }
}

class CarFactory {
    public function createCar() {
        return new Car();
    }
}

$factory = new CarFactory();
for ($i = 0; $i < $n; $i++) {
    $car = $factory->createCar();
    $car->drive();
}
    

This code creates $n car objects using a factory and calls a method on each.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Creating a new Car object inside the loop.
• How many times: Exactly $n times, once per loop iteration.

How Execution Grows With Input

Each time we increase $n, the number of cars created grows the same amount.

Input Size (n)	Approx. Operations
10	10 car creations and drives
100	100 car creations and drives
1000	1000 car creations and drives

Pattern observation: The work grows directly in proportion to the number of objects created.

Final Time Complexity

Time Complexity: O(n)

This means the time to create and use cars grows linearly as we ask for more cars.

Common Mistake

[X] Wrong: "Using a factory makes object creation instant or constant time no matter how many objects."

[OK] Correct: Each object still needs to be created one by one, so time grows with the number of objects.

Interview Connect

Understanding how object creation scales helps you explain design choices clearly and shows you think about efficiency in real code.

Self-Check

"What if the factory cached and reused objects instead of creating new ones each time? How would the time complexity change?"

Practice

1. What is the main purpose of the Factory Pattern in PHP?

easy

A. To create objects without exposing the creation logic to the client

B. To store data in a database

C. To handle user input validation

D. To perform mathematical calculations

Solution

1. Step 1: Understand the role of the Factory Pattern

   The Factory Pattern is designed to create objects while hiding the details of how they are created.

2. Step 2: Compare options with the pattern's purpose

   Only To create objects without exposing the creation logic to the client describes creating objects without exposing creation logic, which matches the Factory Pattern.

3. Final Answer:

   To create objects without exposing the creation logic to the client -> Option A

4. Quick Check:

   Factory Pattern = Object creation hiding [OK]

Hint: Factory pattern hides object creation details [OK]

Common Mistakes:

• Confusing factory pattern with database operations
• Thinking it handles input validation
• Assuming it performs calculations

2. Which of the following is the correct way to define a factory method in PHP?

easy

A. public static function create() {}

B. function create() {}

C. public function create() {}

D. private function create() {}

Solution

1. Step 1: Identify typical factory method signature

   Factory methods are often static so they can be called without creating an instance.

2. Step 2: Check options for static keyword

   Only public static function create() {} uses public static function create(), which is the common factory method style.

3. Final Answer:

   public static function create() {} -> Option A

4. Quick Check:

   Factory method is usually static [OK]

Hint: Factory methods are usually static for easy access [OK]

Common Mistakes:

• Omitting static keyword for factory methods
• Using private visibility which blocks access
• Not specifying visibility (defaults to public but less clear)

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

class Book { }
class ProductFactory {
    public static function create($type) {
        if ($type === 'Book') {
            return new Book();
        }
        return null;
    }
}
$product = ProductFactory::create('Book');
echo get_class($product);

medium

A. null

B. ProductFactory

C. Book

D. Error: Class not found

Solution

1. Step 1: Analyze the factory method call

   The call ProductFactory::create('Book') checks if the type is 'Book' and returns a new Book object.

2. Step 2: Check the output of get_class

   Since the returned object is an instance of Book, get_class($product) returns "Book".

3. Final Answer:

   Book -> Option C

4. Quick Check:

   Factory returns Book object, get_class = Book [OK]

Hint: Factory returns object; get_class shows its class name [OK]

Common Mistakes:

• Expecting factory class name instead of product class
• Assuming null is returned
• Thinking code throws error due to missing class

4. Identify the error in this factory pattern implementation:

class Car {}
class VehicleFactory {
    public static function create($type) {
        if ($type = 'Car') {
            return new Car();
        }
        return null;
    }
}

medium

A. Missing return type declaration

B. Using assignment (=) instead of comparison (==) in the if condition

C. Class Car is not defined

D. Factory method should not be static

Solution

1. Step 1: Check the if condition syntax

   The condition uses = which assigns 'Car' to $type instead of comparing it.

2. Step 2: Understand impact of assignment in condition

   Assignment always returns true, so the if block always runs regardless of input, which is a logic error.

3. Final Answer:

   Using assignment (=) instead of comparison (==) in the if condition -> Option B

4. Quick Check:

   Use == for comparison, not = [OK]

Hint: Use == for comparison, not = assignment in conditions [OK]

Common Mistakes:

• Confusing = and == in if statements
• Thinking static method is wrong here
• Assuming missing return type causes error

5. You want to extend a factory to create different types of vehicles: Car, Bike, and Truck. Which approach best follows the Factory Pattern principles in PHP?

hard

A. Create separate factory classes for CarFactory, BikeFactory, and TruckFactory each with their own create method

B. Use global functions to create Car, Bike, and Truck objects directly

C. Create one class Vehicle with properties to switch between Car, Bike, and Truck behaviors

D. Create a VehicleFactory with a static create method that returns new Car, Bike, or Truck objects based on input

Solution

1. Step 1: Understand the Factory Pattern goal

   The pattern centralizes object creation in one place, hiding details from the client.

2. Step 2: Evaluate options for best factory design

   Create a VehicleFactory with a static create method that returns new Car, Bike, or Truck objects based on input uses one factory class with a method that creates different vehicle objects based on input, which is the classic factory pattern.

3. Step 3: Compare other options

   Create separate factory classes for CarFactory, BikeFactory, and TruckFactory each with their own create method splits factories, which is more like Abstract Factory or multiple factories, not simple Factory Pattern. Create one class Vehicle with properties to switch between Car, Bike, and Truck behaviors mixes behaviors in one class, breaking single responsibility. Use global functions to create Car, Bike, and Truck objects directly uses global functions, which is not object-oriented.

4. Final Answer:

   Create a VehicleFactory with a static create method that returns new Car, Bike, or Truck objects based on input -> Option D

5. Quick Check:

   Single factory method creates varied objects [OK]

Hint: One factory method returns different objects by input [OK]

Common Mistakes:

• Splitting factories unnecessarily
• Mixing multiple behaviors in one class
• Using global functions instead of classes