Understanding SOLID Principles

// BAD: Report does calc + format + persist
// GOOD:
class ReportCalculator { Report compute(Data d) { ... } }
class ReportFormatter  { String toPdf(Report r) { ... } }
class ReportRepository { void save(Report r) { ... } }

interface DiscountPolicy { Money apply(Order o); }
class PricingService {
    private final List<DiscountPolicy> policies;
    Money price(Order o) {
        return policies.stream().map(p -> p.apply(o)).reduce(o.total(), Money::minus);
    }
}

interface Reader<T> { T read(long id); }
interface Writer<T> { void write(T entity); }
// Read-only consumers depend on Reader only.

// Domain owns the interface
package domain;  interface NotificationPort { void send(Message m); }
// Infra implements it
package infra;   class SmtpNotifier implements NotificationPort { ... }