Lesson 11: Abstract Classes vs Interfaces

Abstract classes share partial implementation among related types; interfaces define a contract any unrelated type can fulfill.

Real-time example

A payments app has an interface PaymentMethod implemented by unrelated classes: CreditCard, UPI, Wallet — they share no code, only a contract.

interface PaymentMethod {
    boolean charge(double amount);
}
class UPI implements PaymentMethod {
    public boolean charge(double amount) {
        System.out.println("UPI charged Rs " + amount);
        return true;
    }
}
class CreditCard implements PaymentMethod {
    public boolean charge(double amount) {
        System.out.println("Card charged Rs " + amount);
        return true;
    }
}

What's happening

Any class, regardless of its inheritance tree, can implements PaymentMethod — interfaces cut across unrelated hierarchies.

Rule of thumb: interface for 'can do X', abstract class for 'is a kind of X with shared logic'.