Open/closed

Definition

The open/closed principle says code should be open for extension but closed for modification. You add new behaviour by adding new classes, not by editing code that already works.

Example code

Adding a new discount without touching the checkout

interface Discount {
    int apply(int price);
}

class NoDiscount implements Discount {
    public int apply(int price) { return price; }
}

class FestivalDiscount implements Discount {
    public int apply(int price) { return price * 90 / 100; }
}

class StudentDiscount implements Discount {            // new rule: only a new class was added
    public int apply(int price) { return price * 80 / 100; }
}

class Checkout {
    int total(int price, Discount discount) { return discount.apply(price); }   // never needs to change
}

class Demo {
    public static void main(String[] args) {
        Checkout checkout = new Checkout();
        System.out.println(checkout.total(1_000, new NoDiscount()));
        System.out.println(checkout.total(1_000, new FestivalDiscount()));
        System.out.println(checkout.total(1_000, new StudentDiscount()));
    }
}

// Output:
// 1000
// 900
// 800