Sealed classes

Definition

A sealed class or interface lists exactly which classes may extend it, using permits. The compiler then knows every possible subtype, so a switch over it is checked for completeness without a default.

Example code

A closed set of payment types

sealed interface Payment permits Card, Upi, Cash { }

record Card(String number) implements Payment { }
record Upi(String id) implements Payment { }
record Cash() implements Payment { }

class Demo {
    static String describe(Payment payment) {
        return switch (payment) {                   // no default: the compiler knows all cases
            case Card c -> "Card ending " + c.number().substring(c.number().length() - 4);
            case Upi u -> "UPI " + u.id();
            case Cash c -> "Cash";
        };
    }

    public static void main(String[] args) {
        System.out.println(describe(new Card("1234567812345678")));
        System.out.println(describe(new Upi("asha@bank")));
        System.out.println(describe(new Cash()));
        // adding a new Payment type makes describe() fail to compile until you handle it
    }
}

// Output:
// Card ending 5678
// UPI asha@bank
// Cash