OOP Interview Questions and Answers: Key Concepts with Examples

Prepare for your next interview with these essential Object-Oriented Programming (OOP) questions and answers. Learn about classes, objects, inheritance, access modifiers, and more with clear explanations and code snippets.

Table of Contents

What is OOP?

OOP (Object-Oriented Programming) is a programming paradigm that focuses on creating objects that encapsulate both data and functions. It promotes code reusability, modularity, and a structured approach to programming.

Key Benefits of OOP:

Follows the DRY (Don’t Repeat Yourself) principle
Provides a structured programming approach
Enhances code reusability and maintainability
Offers faster and easier debugging

Difference Between Class and Object

Class: A class is a blueprint or template for creating objects.

class Fruit {
    public $name;
    public function setName($name) {
        $this->name = $name;
    }
}

Object: An instance of a class.

$apple = new Fruit();
$apple->setName("Apple");

What is a Constructor?

A constructor is a special function in a class that initializes object properties when the object is created. It is called automatically upon object instantiation.

class Car {
    public $brand;
    public function __construct($brand) {
        $this->brand = $brand;
    }
}
$bmw = new Car("BMW");

What is a Destructor?

A destructor is called when an object is destroyed or when the script execution ends. It is mainly used for cleanup purposes.

class Test {
    public function __destruct() {
        echo "Object destroyed";
    }
}
$obj = new Test();

Explain Access Modifiers

Access modifiers define the visibility of properties and methods within a class.

Public: Accessible from anywhere.
Protected: Accessible within the class and derived classes.
Private: Accessible only within the class.

Example:

class Example {
    public $publicVar = "Public";
    protected $protectedVar = "Protected";
    private $privateVar = "Private";
}

What is Inheritance?

Inheritance allows a child class to inherit properties and methods from a parent class.

class Fruit {
    public $color;
}
class Apple extends Fruit {
    public function setColor($color) {
        $this->color = $color;
    }
}

What are Class Constants?

Class constants are values that remain unchanged throughout the execution.

class Fruit {
    const COLOR = "Red";
}
echo Fruit::COLOR;

What is an Abstract Class?

An abstract class contains at least one abstract method (declared but not implemented). Child classes must define the abstract method.

abstract class Car {
    abstract public function company();
}
class BMW extends Car {
    public function company() {
        return "BMW";
    }
}

What are Interfaces?

Interfaces define a contract that a class must follow. It ensures that implementing classes have specific methods.

interface Animal {
    public function makeSound();
}
class Cat implements Animal {
    public function makeSound() {
        echo "Meow";
    }
}

Difference Between Interface and Abstract Class

Feature	Abstract Class	Interface
Methods	Can have defined and abstract methods	Only abstract methods
Properties	Can have properties	No properties
Multiple Inheritance	No	Yes

What are PHP Traits?

Traits are used to share methods between multiple classes to avoid code duplication.

trait Logger {
    public function log($message) {
        echo "Log: " . $message;
    }
}
class User {
    use Logger;
}
$user = new User();
$user->log("User logged in");

Static Method vs Static Property

Static methods belong to the class, not an instance.

class Test {
    public static function show() {
        echo "Static Method";
    }
}
Test::show();

Static properties can be accessed without creating an object.

class Counter {
    public static $count = 0;
}
echo Counter::$count;

What is Namespace in PHP?

Namespaces help organize code and prevent class name conflicts.

namespace MyNamespace;
class MyClass {
    public function sayHello() {
        echo "Hello";
    }
}

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