Factory pattern in PHP

Introduction

The factory pattern helps create objects without needing to know the exact class. It makes code easier to change and organize.

When you want to create objects but hide the details of how they are made.

When you have many similar classes and want a simple way to create them.

When you want to change the type of object created without changing the code that uses it.

When you want to keep your code clean and easy to maintain.

When you want to add new types of objects without changing existing code.

Syntax

PHP

<?php

class Factory {
    public static function create($type) {
        switch ($type) {
            case 'A':
                return new ClassA();
            case 'B':
                return new ClassB();
            default:
                throw new Exception('Unknown type');
        }
    }
}

class ClassA {
    public function say() {
        return "I am Class A";
    }
}

class ClassB {
    public function say() {
        return "I am Class B";
    }
}

The factory method is usually static so you can call it without creating the factory object.

Use a switch or if-else to decide which class to create based on input.

Examples

This creates an object of ClassA and calls its say method.

PHP

<?php
$obj = Factory::create('A');
echo $obj->say();

This creates an object of ClassB and calls its say method.

PHP

<?php
$obj = Factory::create('B');
echo $obj->say();

This shows how the factory handles unknown types by throwing an exception.

PHP

<?php
try {
    $obj = Factory::create('C');
} catch (Exception $e) {
    echo $e->getMessage();
}

Sample Program

This program uses the factory to create different vehicle objects. It calls their drive method to show what they do. If an unknown type is requested, it shows an error message.

PHP

<?php

class Factory {
    public static function create($type) {
        switch ($type) {
            case 'Car':
                return new Car();
            case 'Bike':
                return new Bike();
            default:
                throw new Exception('Unknown vehicle type');
        }
    }
}

class Car {
    public function drive() {
        return "Driving a car";
    }
}

class Bike {
    public function drive() {
        return "Riding a bike";
    }
}

try {
    $vehicle1 = Factory::create('Car');
    echo $vehicle1->drive() . "\n";

    $vehicle2 = Factory::create('Bike');
    echo $vehicle2->drive() . "\n";

    $vehicle3 = Factory::create('Plane');
    echo $vehicle3->drive() . "\n";
} catch (Exception $e) {
    echo $e->getMessage() . "\n";
}

Important Notes

The factory pattern helps keep your code flexible and easy to update.

It separates object creation from usage, so you can add new classes without changing much code.

Always handle unknown types to avoid errors.

Summary

The factory pattern creates objects without exposing the creation logic.

It helps manage many related classes with a simple interface.

Use it to keep your code clean and easy to maintain.

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