Understanding Object-Oriented Principles

public final class Account {
    private long balanceCents;
    public long getBalanceCents() { return balanceCents; }
    public void deposit(long cents) {
        if (cents <= 0) throw new IllegalArgumentException("positive only");
        this.balanceCents += cents;
    }
}

public interface PaymentGateway {
    PaymentResult charge(Money amount, Card card);
}
// Callers depend only on the contract, not Stripe/PayPal details.

List<Shape> shapes = List.of(new Circle(2), new Square(3));
double total = shapes.stream().mapToDouble(Shape::area).sum();

// BAD: class CachingUserService extends UserService { ... }
// GOOD:
public class CachingUserService implements UserService {
    private final UserService delegate;
    private final Cache<Long, User> cache;
    public User find(long id) {
        return cache.get(id, delegate::find);
    }
}

// BAD: order.getCustomer().getAddress().getStreet()
// GOOD:
order.shipTo(label);  // order knows how to expose what's needed