Polymorphism

Compile-time and runtime polymorphism, method overloading and overriding in Java.

Polymorphism means “many forms.” It allows objects of different classes to be treated through the same interface.

Compile-Time Polymorphism (Static Binding)

Resolved at compile time through:

• Method overloading — Multiple methods with the same name but different parameter lists.
• Constructor overloading — Multiple constructors with different parameters.

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

Runtime Polymorphism (Dynamic Binding)

Resolved at runtime through method overriding — a subclass provides its own implementation of a superclass method.

The key mechanism is using a superclass reference to point to a subclass object:

Shape s = new Circle();  // superclass reference, subclass object
s.draw();                // calls Circle's draw() at runtime

Example

// PolymorphismDemo.java
// Demonstrates compile-time and runtime polymorphism in Java.

// Superclass
class Shape {
	void draw() {
		System.out.println("Drawing a shape");
	}
}

// Subclass 1
class Circle extends Shape {
	void draw() {
		System.out.println("Drawing a circle");
	}
}

// Subclass 2
class Square extends Shape {
	void draw() {
		System.out.println("Drawing a square");
	}
}

public class PolymorphismDemo {
	public static void main(String[] args) {
		// Superclass reference to subclass objects
		Shape s1 = new Circle();
		Shape s2 = new Square();
		s1.draw(); // Calls Circle's draw()
		s2.draw(); // Calls Square's draw()
	}
}

Full source: examples/10-polymorphism/PolymorphismDemo.java