Java Programming Lesson - 12 | Inheritance & Polymorphism — The Heart of Object-Oriented Programming in Java

Java Programming Lesson - 12

Welcome to Lesson-12: Inheritance & Polymorphism — the heart of Object-Oriented Programming in Java.

These concepts allow us to reuse code and write flexible, scalable programs.

Object-Oriented Programming (OOP) is one of the most powerful programming paradigms in modern software development. Among its core principles, Inheritance and Polymorphism stand out as the true engines that drive code flexibility, reusability, and maintainability.

In Java, these two concepts work together to create cleaner, more modular, and easily scalable programs — making them essential knowledge for every Java developer.

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📘 Lesson-12 — Inheritance & Polymorphism

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✅ 1️⃣ What is Inheritance?

Inheritance allows one class (child/subclass) to acquire the properties and behaviors (fields and methods) of another class (parent/superclass).

Why Inheritance Matters

• Eliminates code duplication
• Improves code organization
• Supports hierarchical classifications
• Makes code easy to maintain or extend

• Parent Class (Super Class) → Base class
• Child Class (Sub Class) → Derived class

Syntax:

class Parent {
    int x = 10;
    void show() {
        System.out.println("Parent method");
    }
}

class Child extends Parent {
    void display() {
        System.out.println("Child method");
    }
}

Example:

public class Main {
    public static void main(String[] args) {
        Child c = new Child();
        System.out.println("x = " + c.x); // inherited
        c.show(); // inherited
        c.display(); // own method
    }
}

✅ Output:

x = 10
Parent method
Child method

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✅ 2️⃣ Types of Inheritance in Java

Type	Description
Single	One child inherits one parent
Multilevel	Grandparent → Parent → Child
Hierarchical	One parent → multiple children

📌 Java does NOT support multiple inheritance with classes directly (use interfaces instead)

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✅ 3️⃣ What is Polymorphism?

Polymorphism = "Many Forms"

• Allows objects to behave differently based on context

Types:

1. Compile-time / Method Overloading
2. Runtime / Method Overriding

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✅ 3.1 Method Overloading (Compile-time)

Same method name → Different parameters

class Calculator {
    int add(int a, int b) {
        return a + b;
    }

    int add(int a, int b, int c) {
        return a + b + c;
    }
}

public class Main {
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        System.out.println(calc.add(5, 10));      // 15
        System.out.println(calc.add(5, 10, 15));  // 30
    }
}

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✅ 3.2 Method Overriding (Runtime)

Child class redefines a method of parent class

class Animal {
    void sound() {
        System.out.println("Animal makes sound");
    }
}

class Dog extends Animal {
    void sound() {  // overriding
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal a = new Animal();
        a.sound(); // Animal makes sound

        Dog d = new Dog();
        d.sound(); // Dog barks
    }
}

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✅ Key Points

✔ Inheritance → code reuse
✔ Polymorphism → flexibility / multiple forms
✔ Overloading → Compile-time
✔ Overriding → Runtime

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📝 Exercise for You ✅

Create a program:

1️⃣ Parent class: Shape → method area()
2️⃣ Child classes: Circle (radius), Rectangle (length, width)
3️⃣ Override area() in both children → print area
4️⃣ Demonstrate polymorphism by calling area() for each object

Example Output:

Circle area: 78.5
Rectangle area: 50

Reply with:
✅ Code
✅ Output

I will check & give feedback 🔥

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🎯 Quick Quiz

1️⃣ Which keyword is used to inherit a class?
A) implements
B) extends
C) inherit

2️⃣ Polymorphism allows:
A) One form only
B) Multiple forms
C) No forms

Reply like:
1 → ?, 2 → ?

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Conclusion

Inheritance and Polymorphism are not just OOP concepts — they are the backbone of how Java applications are structured and extended. By mastering these two principles, you unlock the ability to write clean, flexible, and future-proof code.

Whether you're preparing for interviews, building real-world projects, or improving your Java fundamentals, understanding these concepts deeply will make you a stronger developer.

✅ Lesson-12 Completed!

✅ NEXT PART - Advanced Java Topics — this is where you go from “Java beginner” to pro-level developer.

📘 Advanced Java Topics Roadmap

1️⃣ Collections Framework

• Lists (ArrayList, LinkedList)
• Sets (HashSet, TreeSet)
• Maps (HashMap, TreeMap)
• Iterators & Loops

2️⃣ Exception Handling Deep Dive

• Custom Exceptions
• Multiple catch, try-with-resources

3️⃣ Multithreading & Concurrency

• Thread class & Runnable interface
• Thread states, sleep, join
• Synchronization

4️⃣ File I/O

• Reading & Writing Files (Text/Binary)
• BufferedReader, BufferedWriter
• File class & Paths

5️⃣ Java Generics

• Type-safe Collections
• Generic methods & classes

6️⃣ Java Streams & Lambda Expressions

• Functional programming in Java
• Filtering, mapping, reducing collections

7️⃣ Java Networking (Optional Advanced)

• Sockets, HTTP requests
• Client-Server programming

Do You Ready To Advanced JAVA PROGRAMMING Learn Comment Under The Post.

Frequently Asked Questions

What is inheritance in Java?

• Inheritance allows a child class to use parent class features.
• Supports reusability and reduces duplicate code.

What is method overriding?

• Subclass redefines a parent class method.
• Used to achieve runtime polymorphism.

What is polymorphism?

• One method behaving differently based on the object.
• Types: Compile-time and Runtime.