design_patterns.md

Singleton Pattern

public class FinancialValidator {
    private static FinancialValidator instance;

    private FinancialValidator() {
        recentlyCheckedRequestIds = new ArrayList<>();
    }

    public static FinancialValidator getInstance() {
        if (instance == null) {
            instance = new FinancialValidator();
        }
        return instance;
    }
}

Iterator Pattern

public abstract class AbstractIterator<T> {
    public abstract T next();
    public abstract int getSteps();
    public abstract void setSteps(int step);
    public abstract boolean isDone();
    public abstract T current();
    public abstract T first();
}

public class TicketTypeIterator extends AbstractIterator<TicketType> {
    private Event event;
    private int currentIndex;
    private List<TicketType> ticketTypes;
    
    public TicketTypeIterator(Event event) {
        this.event = event;
        this.ticketTypes = event.getTicketTypes();
        this.currentIndex = 0;
    }

    @Override
    public TicketType next() {
        if (isDone()) {
            throw new IllegalStateException("No more ticket types");
        }
        TicketType ticketType = ticketTypes.get(currentIndex);
        currentIndex++;
        return ticketType;
    }

    // other overridden methods...
}

Template Method

public abstract class AbstractRecommenderModel {
    public abstract List<Event> recommend(int n);
    public abstract List<Event> recommend();
}

public class NlpRecommenderModel extends AbstractRecommenderModel {
    @Override
    public List<Event> recommend(int n) {
        // implementation...
    }

    @Override
    public List<Event> recommend() {
        // implementation...
    }
}

public class PopularityRecommenderModel extends AbstractRecommenderModel {
    @Override
    public List<Event> recommend(int n) {
        // implementation...
    }

    @Override
    public List<Event> recommend() {
        // implementation...
    }
}

Facade

public class EventRecommender {
    private int id;
    private User user;
    private AbstractRecommenderModel model;
    private int numberRecommendations;
    private List<Event> eventRecommendations;

    public EventRecommender(AbstractRecommenderModel model) {
        this.user = model.getUser();
        this.model = model;
        this.numberRecommendations = 3;
        this.eventRecommendations = new ArrayList<>();
    }

    public List<Event> sendRecommendation() {
        return eventRecommendations;
    }

    public void setNumberRecommendations(int numberRecommendations) {
        this.numberRecommendations = numberRecommendations;
    }

    // ...
}

Strategy

public abstract class PaymentMethod {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    protected int id;

    // common fields if any

    // Getters and Setters
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }
}

public class CardPayment extends PaymentMethod {
    @Column(nullable = false)
    private String cardNumber;

    @Column(nullable = false)
    private String cardHolder;

    @Temporal(TemporalType.DATE)
    @Column(nullable = false)
    private Date expirationDate;

    @Column(nullable = false)
    private String cvv;

    public CardPayment() {
    }

    public CardPayment(String cardNumber, String cardHolder, Date expirationDate, String cvv) {
        this.cardNumber = cardNumber;
        this.cardHolder = cardHolder;
        this.expirationDate = expirationDate;
        this.cvv = cvv;
    }

    // ...
}    

public class CheckingAccountPayment extends PaymentMethod {
    @Column(nullable = false)
    private String accountNumber;

    @Column(nullable = false)
    private String routingNumber;

    public CheckingAccountPayment() {
    }

    public CheckingAccountPayment(String accountNumber, String routingNumber) {
        this.accountNumber = accountNumber;
        this.routingNumber = routingNumber;
    }
    
    // ...
}