What is Polymorphism?

Polymorphism means "many forms." In Java, it refers to the ability of a single reference type (usually a parent class or interface) to refer to objects of different subtypes.

Java supports two main types of polymorphism:

Compile-time (Static): Achieved via method overloading

Run-time (Dynamic): Achieved via method overriding

Compile-time Polymorphism (Method Overloading)

Same method name, different parameter lists in the same class.

class Printer {
    void print(String msg) {
        System.out.println(msg);
    }

    void print(int num) {
        System.out.println("Number: " + num);
    }
}

Usage:

Printer p = new Printer();
p.print("Hello");
p.print(123);

Run-time Polymorphism (Method Overriding)

A subclass provides its own version of a method already defined in its superclass.

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

class Dog extends Animal {
    @Override
    void makeSound() {
        System.out.println("Dog barks");
    }
}

Dynamic Method Dispatch

A parent reference can refer to a child object, and the overridden method will be called at runtime.

Animal a = new Dog();
a.makeSound(); // Output: Dog barks

Even though the reference is of type Animal, the actual object is a Dog.

What Cannot Be Overridden?

• final methods

• static methods (they are hidden, not overridden)

• Constructors

Why Use Polymorphism?

• Makes code flexible and extensible

• Enables late binding and dynamic behavior

• Helps you program to an interface, not implementation

Summary

• Polymorphism allows one interface to be used for a variety of object types.

• Overloading: same class, different parameters.

• Overriding: subclass redefines a parent’s method.

• Enables you to write flexible and reusable code.

Assignment: Lesson 5 – Polymorphism

Objective:

Understand and apply method overloading and overriding.

Part 1: Conceptual Questions

Answer these in comments or a separate file:

1. What is polymorphism in Java?

2. What is the difference between overloading and overriding?

3. Why is runtime polymorphism useful?

Part 2: Coding Task

Step 1 – Overloading:

Create a class Calculator with two methods:

• add(int a, int b) returns the sum of two integers.

• add(double a, double b) returns the sum of two doubles.

Step 2 – Overriding:

Create a class Shape with a method area() that prints "Calculating area".
Then create a class Circle that extends Shape and overrides area() to print "Area of Circle = πr²".

In the main() method:

• Call both overloaded add() methods.

• Use a Shape reference to call area() on a Circle object.

Github: https://github.com/tuanbeovnn/java-bootcamp/tree/java-bootcamp-oop-chapter5