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.

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📘 Lesson-12 — Inheritance & Polymorphism

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✅ 1️⃣ What is Inheritance?

Inheritance allows one class to acquire the properties and methods of another class.

• 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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✅ 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.