III. Polymorphism

Polymorphism in Java

Polymorphism is one of the four core principles of Object-Oriented Programming (OOP), alongside Encapsulation, Inheritance, and Abstraction.

Definition: Polymorphism allows objects of different classes to be treated as objects of a common superclass. The correct method is selected at runtime based on the actual object type.

Polymorphism means “many forms.” In Java, it lets the same method name or object type behave differently depending on context.

Benefits of Polymorphism

• Improves code flexibility and reusability
• Supports extensibility without changing existing code
• Enables dynamic method binding (runtime decision-making)

Types of Polymorphism in Java:

Type	Description	Example
Compile-time (Static)	Method Overloading	print(int) vs print(String)
Runtime (Dynamic)	Method Overriding using Inheritance	Animal a = new Dog(); a.speak();

A. Overloading and Overriding

These are two fundamental techniques used to implement polymorphism in Java. While they may sound similar, they are very different in how and when they are applied.

1. Method Overloading (Compile-Time Polymorphism)

Definition: Overloading means having multiple methods in the same class with the same name but different parameter lists.

Characteristics:

• Happens within the same class.
• Parameters must differ by type, number, or order.
• Return type can be the same or different.
• Happens at compile time.

Example:

class Printer {
    void print(String s) {
        System.out.println("String: " + s);
    }

    void print(int n) {
        System.out.println("Integer: " + n);
    }

    void print(String s, int n) {
        System.out.println("String & Integer: " + s + ", " + n);
    }
}

Usage:

Printer p = new Printer();
p.print("Hello");     // calls print(String)
p.print(42);          // calls print(int)
p.print("Age", 30);   // calls print(String, int)

2. Method Overriding (Runtime Polymorphism)

Definition: Overriding is when a subclass provides its own version of a method already defined in its superclass.

Characteristics:

• Must have same method name, parameters, and return type (or covariant).
• Must be in a subclass.
• The method must be inherited, not private.
• Uses @Override annotation (recommended).
• Happens at runtime.

Example:

class Animal {
    void speak() {
        System.out.println("Some animal sound");
    }
}

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

Usage:

Animal a = new Dog();
a.speak(); // Bark — Dog's version overrides Animal's

3. Comparison Table

Feature	Overloading	Overriding
Location	Same class	Subclass (inherits from superclass)
Parameters	Must differ	Must be identical
Return type	Can differ	Must be same (or covariant)
Timing	Compile-time	Runtime
Access modifier	Can be any	Cannot reduce visibility
Static methods	Yes	No (static methods are not overridden)
Annotation	No. Optional	Yes. @Override (strongly recommended)

Notes:

• Constructor Overloading is allowed, but constructors cannot be overridden.
• Static Methods: Can be overloaded, but not overridden (they are class-level, not instance-level).
• Final Methods: Cannot be overridden.
• Private Methods: Cannot be overridden (they are not inherited).