# 📚 Java Programming Language – Part 5/10: Inheritance & Polymorphism
#Java #OOP #Inheritance #Polymorphism #Programming

Welcome to Part 5 of our Java series! Today we'll explore two fundamental OOP concepts: Inheritance and Polymorphism.

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## 🔹 Inheritance in Java
Inheritance allows a class to acquire properties and methods of another class.

### 1. Basic Inheritance Syntax

// Parent class (Superclass)
class Vehicle {
    String brand;
    
    public void start() {
        System.out.println("Vehicle starting...");
    }
}

// Child class (Subclass)
class Car extends Vehicle {  // 'extends' keyword
    int numberOfDoors;
    
    public void honk() {
        System.out.println("Beep beep!");
    }
}

// Usage:
Car myCar = new Car();
myCar.brand = "Toyota";  // Inherited from Vehicle
myCar.start();          // Inherited method
myCar.honk();           // Child's own method

### 2. Inheritance Types
Java supports:
- Single Inheritance (One parent → one child)
- Multilevel Inheritance (Grandparent → parent → child)
- Hierarchical Inheritance (One parent → multiple children)

*Note: Java doesn't support multiple inheritance with classes*

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## 🔹 Method Overriding
Subclass can provide its own implementation of an inherited method.

class Vehicle {
    public void start() {
        System.out.println("Vehicle starting...");
    }
}

class ElectricCar extends Vehicle {
    @Override  // Annotation (optional but recommended)
    public void start() {
        System.out.println("Electric car starting silently...");
    }
}

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## 🔹 super Keyword
Used to access superclass members from subclass.

### 1. Accessing Superclass Methods

class ElectricCar extends Vehicle {
    @Override
    public void start() {
        super.start();  // Calls Vehicle's start()
        System.out.println("Battery check complete");
    }
}

### 2. Superclass Constructor

class Vehicle {
    String brand;
    
    public Vehicle(String brand) {
        this.brand = brand;
    }
}

class Car extends Vehicle {
    int doors;
    
    public Car(String brand, int doors) {
        super(brand);  // Must be first statement
        this.doors = doors;
    }
}

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## 🔹 Polymorphism
"Many forms" - ability of an object to take many forms.

### 1. Compile-time Polymorphism (Method Overloading)

class Calculator {
    // Same method name, different parameters
    int add(int a, int b) { return a + b; }
    double add(double a, double b) { return a + b; }
}

### 2. Runtime Polymorphism (Method Overriding)

Vehicle v1 = new Vehicle();  // Parent reference, parent object
Vehicle v2 = new Car();      // Parent reference, child object

v1.start();  // Calls Vehicle's start()
v2.start();  // Calls Car's start() if overridden

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## 🔹 final Keyword
Restricts inheritance and overriding.

final class CannotBeExtended { }  // Cannot be inherited

class Parent {
    final void cannotOverride() { }  // Cannot be overridden
}

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## 🔹 Object Class
All classes implicitly extend Java's Object class.

Important methods:
- toString() - String representation
- equals() - Compare objects
- hashCode() - Hash code value

class MyClass { }  // Automatically extends Object

MyClass obj = new MyClass();
System.out.println(obj.toString());  // Outputs something like MyClass@1dbd16a6

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