What Is The Inheritance In Object-oriented Programming?

Object-oriented programming (OOP) is a powerful paradigm that allows developers to model real-world entities in a modular and organized manner.

One of the fundamental concepts in OOP is inheritance, which provides a mechanism for creating a new class based on an existing class. For example:

Suppose you have two classes ClassA and ClassB, and inside the “ClassB“, you want to use some or all properties and methods of the “ClassA“, and you can do this with the help of Inheritance.

This article will explore the principles of inheritance, its syntax, and its significance in building scalable and maintainable software systems.

Table of Contents #
  1. The Essence of Inheritance
  2. Basic Syntax of Inheritance
  3. Types of Inheritance
  4. Access Modifiers in Inheritance
  5. Method Overriding
  6. Advantages of Inheritance
  7. Best Practices
  8. Conclusion

1. The Essence of Inheritance

Inheritance is a concept that reflects the hierarchical nature of relationships between objects.

It enables the creation of a new class, referred to as the derived class or child class, by inheriting properties and behaviors from an existing class known as the base class or parent class.

The key idea is to reuse and extend the functionality of the parent class, allowing for code reusability, abstraction, and the modeling of a hierarchy of classes.

2. Basic Syntax of Inheritance

In many object-oriented languages, including PHP, the extends keyword is used to declare inheritance. Let’s examine a simple example to illustrate the syntax:

class Animal {
    public $species;

    public function speak() {
        echo "Animal speaks!";
    }
}

class Dog extends Animal {
    public function bark() {
        echo "Woof! Woof!";
    }
}
  1. In this example, the Dog class extends the Animal class.
  2. The “Dog” class inherits the species property and the speak method from the “Animal” class.
  3. Additionally, the “Dog” class introduces its own method, bark.
  4. Now, an instance of “Dog” can both “bark” and “speak”, inheriting the common traits of an animal.

3. Types of Inheritance

  1. Single Inheritance:

    A class can inherit from only one class. For example, Class B extends Class A.

    Single Inheritance example
  2. Multiple Inheritance (not supported in PHP):

    If a Child class inherits more than one parent class is called Multiple inheritance. Some languages do not support multiple inheritance, such as Java, PHP, etc.

    Multiple Inheritance Example
  3. Multilevel Inheritance:

    When more than two classes are inherited as a chain, it is called multilevel inheritance. In this type of inheritance, a child class may have its own child classes.

    Multilevel Inheritance example
  4. Hierarchical Inheritance:

    When a parent class has multiple child classes is called Hierarchical inheritance.

    Hierarchical Inheritance example
  5. Hybrid Inheritance:

    A combination of two or more types of inheritance is called Hybrid Inheritance.

    This following Hybrid Inheritance example contains “Multilevel inheritance” as well as “Hierarchical inheritance”.

    Hybrid Inheritance example

4. Access Modifiers in Inheritance

Access modifiers, such as public, protected, and private, determine the visibility of properties and methods in both parent and child classes.

  • Public: Public members are accessible everywhere, both in the parent and child classes.
  • Protected: Protected members are visible in the parent class and its subclasses but not accessible outside.
  • Private: Private members are only accessible within the declaring class.
class ParentClass {
    public $publicProperty;
    protected $protectedProperty;
    private $privateProperty;

    public function publicMethod() {}
    protected function protectedMethod() {}
    private function privateMethod() {}
}

class ChildClass extends ParentClass {
    // Inherited properties and methods

    // Additional properties and methods
    public function childMethod() {
        $this->publicProperty;      // Accessible
        $this->protectedProperty;   // Accessible
        // $this->privateProperty;   // Not accessible in ChildClass

        $this->publicMethod();      // Accessible
        $this->protectedMethod();   // Accessible
        // $this->privateMethod();   // Not accessible in ChildClass
    }
}

5. Method Overriding

Inheritance allows child classes to provide a specific implementation for a method that is already defined in the parent class. This process is known as method overriding.

The child class provides its own version of the method, which takes precedence when called on an instance of the child class.

class Animal {
    public function speak() {
        echo "Animal speaks!";
    }
}

class Dog extends Animal {
    public function speak() {
        echo "Dog barks!";
    }
}

$animal = new Animal();
$dog = new Dog();

$animal->speak(); // Output: Animal speaks!
$dog->speak();    // Output: Dog barks!

6. Advantages of Inheritance

  1. Code Reusability: Inheritance promotes the reuse of code from existing classes, reducing redundancy and promoting a more efficient development process.
  2. Abstraction: Base classes can represent abstract concepts, while derived classes provide concrete implementations. This abstraction helps in designing more scalable and maintainable systems.
  3. Extensibility: New functionality can be added to a system without modifying existing code. Derived classes can extend or override the behavior of the base classes.
  4. Hierarchy and Organization: Inheritance allows the creation of a hierarchy of classes that mirrors real-world relationships, making the codebase more intuitive and easier to understand.

7. Best Practices

  1. Favor Composition Over Inheritance: While inheritance is a powerful tool, it can lead to complex hierarchies. In some cases, composition (object containment) might be a more flexible and maintainable approach.
  2. Avoid Deep Hierarchies: Excessive levels of inheritance can make the codebase difficult to understand and maintain. Aim for a balance between abstraction and simplicity.
  3. Use Access Modifiers Wisely: Choose access modifiers carefully based on whether a property or method should be accessible in child classes or remain private to the parent class.
  4. Follow the Liskov Substitution Principle (LSP): Derived classes should be substitutable for their base classes without affecting the correctness of the program.

8. Conclusion

Inheritance is a cornerstone of object-oriented programming, providing a mechanism for creating relationships between classes and promoting code reuse and abstraction.

Understanding how to design and use inheritance effectively contributes to the creation of modular, extensible, and maintainable software systems.

By following best practices and leveraging the power of inheritance, developers can design robust and scalable applications that reflect the complexities of the real world.