OOP: Polymorphism & Interfaces

Java supports two abstraction frameworks:

• Abstract Class: A class declared `abstract` that cannot be instantiated. Can contain constructor fields, instance states, and fully defined methods alongside abstract method signatures.
• Interface: A contract definition. Interfaces contain no instance fields (only static final constants) and by default specify abstract signatures. Classes implement interfaces using the `implements` keyword. A class can implement multiple interfaces.

When you reference a child object using a parent type, Java decides which method implementation to run at runtime based on the actual object type, not the reference type. Let's observe this pattern:

interface Drivable {
    void drive(); // Interface abstract method
}

class Car implements Drivable {
    @Override
    public void drive() {
        System.out.println("Car is driving on roads.");
    }
}

class Boat implements Drivable {
    @Override
    public void drive() {
        System.out.println("Boat is cruising on water.");
    }
}

public class Main {
    public static void main(String[] args) {
        // Polymorphism: Reference type is the interface, object is concrete child
        Drivable v1 = new Car();
        Drivable v2 = new Boat();

        // Dynamic Method Dispatch determines execution at runtime
        v1.drive();
        v2.drive();
    }
}