Inheritance in Java with Example Programs (PDF)

Unlock the Secrets of Reusability and Extensibility in Your Java Applications

Table of Contents

What is Inheritance?

Imagine a family tree. Parents pass down traits to their children, who inherit those characteristics while also developing their own unique qualities.

In Java programming, inheritance works similarly. It’s a fundamental pillar of object-oriented programming (OOP) that enables you to create new classes (child classes or subclasses) based on existing classes (parent classes or superclasses).

This fosters code reusability, eliminates redundancy, and promotes a logical organization of your software’s structure.

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Why Inheritance Matters

Code Reusability: No need to reinvent the wheel. Reuse fields (variables) and methods from parent classes in your child classes.
Extensibility: Easily extend the functionality of existing classes without modifying the original code.
Readability: Inheritance mirrors real-world relationships, making your code easier to understand.
Polymorphism: Treat objects of different classes interchangeably, enhancing flexibility.

How Inheritance Works: The Basics

The extends Keyword:
- In Java, you use the extends keyword to establish the inheritance relationship.

class Animal {  // Parent class (Superclass)
   // ... (attributes and methods)
}

class Dog extends Animal {  // Child class (Subclass)
   // ... (additional attributes and methods)
}

Constructors and Inheritance:
- Child classes don’t inherit constructors directly from their parent classes.
- Use the super() keyword within the child class constructor to invoke the parent class constructor.
Overriding Methods:
- A child class can provide its own implementation of a method inherited from the parent class. This is called method overriding.
- Overriding lets you customize the behavior of inherited methods.

Illustrative Example: Vehicles

class Vehicle { // Parent class
    String brand;
    int year;

    public void start() {
        System.out.println("The vehicle starts.");
    }
}

class Car extends Vehicle { // Child class
    int numDoors;

    public void openTrunk() {
        System.out.println("The trunk opens.");
    }
}

// Usage
Car myCar = new Car();
myCar.brand = "Toyota"; 
myCar.year = 2024;
myCar.numDoors = 4;

myCar.start();    // Inherited from Vehicle
myCar.openTrunk(); // Specific to Car

Types of Inheritance in Java

Single Inheritance: One class inherits from a single parent class.
Multilevel Inheritance: A chain of inheritance where Class C inherits from Class B, which inherits from Class A.
Hierarchical Inheritance: Multiple classes inherit from a common parent class.

Note: Java does not support multiple inheritance (inheriting from multiple parent classes) directly with classes, but it does through interfaces.