Special Case

Propósito

Define algunos casos especiales, y los encapsula en subclases que proporcionan diferentes comportamientos especiales.

Explicación

Ejemplo del mundo real

En un sistema de comercio electrónico, la capa de presentación espera que la capa de aplicación produzca cierto modelo de vista.
Tenemos un escenario de éxito, en el que el modelo de vista de recibo contiene datos reales de la compra,
y un par de escenarios de fracaso.

En otras palabras

El patrón Special Case permite devolver objetos reales no nulos que realizan comportamientos especiales.

En Patterns of Enterprise Application Architecture se dice
la diferencia con el Patrón de Objeto Nulo

Si me perdonas el juego de palabras irresistible, yo veo el Objeto Nulo como un caso especial de Caso Especial.

Ejemplo programático

Para centrarnos en el patrón en sí, implementamos la BD y el bloqueo de mantenimiento del sistema de comercio electrónico mediante la instancia singleton.

public class Db {
  private static Db instance;
  private Map<String, User> userName2User;
  private Map<User, Account> user2Account;
  private Map<String, Product> itemName2Product;

  public static Db getInstance() {
    if (instance == null) {
      synchronized (Db.class) {
        if (instance == null) {
          instance = new Db();
          instance.userName2User = new HashMap<>();
          instance.user2Account = new HashMap<>();
          instance.itemName2Product = new HashMap<>();
        }
      }
    }
    return instance;
  }

  public void seedUser(String userName, Double amount) {
    User user = new User(userName);
    instance.userName2User.put(userName, user);
    Account account = new Account(amount);
    instance.user2Account.put(user, account);
  }

  public void seedItem(String itemName, Double price) {
    Product item = new Product(price);
    itemName2Product.put(itemName, item);
  }

  public User findUserByUserName(String userName) {
    if (!userName2User.containsKey(userName)) {
      return null;
    }
    return userName2User.get(userName);
  }

  public Account findAccountByUser(User user) {
    if (!user2Account.containsKey(user)) {
      return null;
    }
    return user2Account.get(user);
  }

  public Product findProductByItemName(String itemName) {
    if (!itemName2Product.containsKey(itemName)) {
      return null;
    }
    return itemName2Product.get(itemName);
  }

  public class User {
    private String userName;

    public User(String userName) {
      this.userName = userName;
    }

    public String getUserName() {
      return userName;
    }

    public ReceiptDto purchase(Product item) {
      return new ReceiptDto(item.getPrice());
    }
  }

  public class Account {
    private Double amount;

    public Account(Double amount) {
      this.amount = amount;
    }

    public MoneyTransaction withdraw(Double price) {
      if (price > amount) {
        return null;
      }
      return new MoneyTransaction(amount, price);
    }

    public Double getAmount() {
      return amount;
    }
  }

  public class Product {
    private Double price;

    public Product(Double price) {
      this.price = price;
    }

    public Double getPrice() {
      return price;
    }
  }
}

public class MaintenanceLock {
  private static final Logger LOGGER = LoggerFactory.getLogger(MaintenanceLock.class);

  private static MaintenanceLock instance;
  private boolean lock = true;

  public static MaintenanceLock getInstance() {
    if (instance == null) {
      synchronized (MaintenanceLock.class) {
        if (instance == null) {
          instance = new MaintenanceLock();
        }
      }
    }
    return instance;
  }

  public boolean isLock() {
    return lock;
  }

  public void setLock(boolean lock) {
    this.lock = lock;
    LOGGER.info("Maintenance lock is set to: " + lock);
  }
}

Primero presentaremos la capa de presentación, la interfaz del modelo de vista de recibo y su implementación de un escenario exitoso.

public interface ReceiptViewModel {
  void show();
}

public class ReceiptDto implements ReceiptViewModel {

  private static final Logger LOGGER = LoggerFactory.getLogger(ReceiptDto.class);

  private Double price;

  public ReceiptDto(Double price) {
    this.price = price;
  }

  public Double getPrice() {
    return price;
  }

  @Override
  public void show() {
    LOGGER.info("Receipt: " + price + " paid");
  }
}

Y aquí están las implementaciones de los escenarios de fracaso, que son los casos especiales.

public class DownForMaintenance implements ReceiptViewModel {
  private static final Logger LOGGER = LoggerFactory.getLogger(DownForMaintenance.class);

  @Override
  public void show() {
    LOGGER.info("Down for maintenance");
  }
}

public class InvalidUser implements ReceiptViewModel {
  private static final Logger LOGGER = LoggerFactory.getLogger(InvalidUser.class);

  private final String userName;

  public InvalidUser(String userName) {
    this.userName = userName;
  }

  @Override
  public void show() {
    LOGGER.info("Invalid user: " + userName);
  }
}

public class OutOfStock implements ReceiptViewModel {

  private static final Logger LOGGER = LoggerFactory.getLogger(OutOfStock.class);

  private String userName;
  private String itemName;

  public OutOfStock(String userName, String itemName) {
    this.userName = userName;
    this.itemName = itemName;
  }

  @Override
  public void show() {
    LOGGER.info("Out of stock: " + itemName + " for user = " + userName + " to buy");
  }
}

public class InsufficientFunds implements ReceiptViewModel {
  private static final Logger LOGGER = LoggerFactory.getLogger(InsufficientFunds.class);

  private String userName;
  private Double amount;
  private String itemName;

  public InsufficientFunds(String userName, Double amount, String itemName) {
    this.userName = userName;
    this.amount = amount;
    this.itemName = itemName;
  }

  @Override
  public void show() {
    LOGGER.info("Insufficient funds: " + amount + " of user: " + userName
        + " for buying item: " + itemName);
  }
}

En segundo lugar, está la capa de aplicación, la implementación de los servicios de aplicación y la implementación de los servicios de dominio.

public class ApplicationServicesImpl implements ApplicationServices {
  private DomainServicesImpl domain = new DomainServicesImpl();

  @Override
  public ReceiptViewModel loggedInUserPurchase(String userName, String itemName) {
    if (isDownForMaintenance()) {
      return new DownForMaintenance();
    }
    return this.domain.purchase(userName, itemName);
  }

  private boolean isDownForMaintenance() {
    return MaintenanceLock.getInstance().isLock();
  }
}

public class DomainServicesImpl implements DomainServices {
  public ReceiptViewModel purchase(String userName, String itemName) {
    Db.User user = Db.getInstance().findUserByUserName(userName);
    if (user == null) {
      return new InvalidUser(userName);
    }

    Db.Account account = Db.getInstance().findAccountByUser(user);
    return purchase(user, account, itemName);
  }

  private ReceiptViewModel purchase(Db.User user, Db.Account account, String itemName) {
    Db.Product item = Db.getInstance().findProductByItemName(itemName);
    if (item == null) {
      return new OutOfStock(user.getUserName(), itemName);
    }

    ReceiptDto receipt = user.purchase(item);
    MoneyTransaction transaction = account.withdraw(receipt.getPrice());
    if (transaction == null) {
      return new InsufficientFunds(user.getUserName(), account.getAmount(), itemName);
    }

    return receipt;
  }
}

Por último, el cliente envía solicitudes a los servicios de la aplicación para obtener la vista de la presentación.

    // DB seeding
    LOGGER.info("Db seeding: " + "1 user: {\"ignite1771\", amount = 1000.0}, "
        + "2 products: {\"computer\": price = 800.0, \"car\": price = 20000.0}");
    Db.getInstance().seedUser("ignite1771", 1000.0);
    Db.getInstance().seedItem("computer", 800.0);
    Db.getInstance().seedItem("car", 20000.0);

    var applicationServices = new ApplicationServicesImpl();
    ReceiptViewModel receipt;

    LOGGER.info("[REQUEST] User: " + "abc123" + " buy product: " + "tv");
    receipt = applicationServices.loggedInUserPurchase("abc123", "tv");
    receipt.show();
    MaintenanceLock.getInstance().setLock(false);
    LOGGER.info("[REQUEST] User: " + "abc123" + " buy product: " + "tv");
    receipt = applicationServices.loggedInUserPurchase("abc123", "tv");
    receipt.show();
    LOGGER.info("[REQUEST] User: " + "ignite1771" + " buy product: " + "tv");
    receipt = applicationServices.loggedInUserPurchase("ignite1771", "tv");
    receipt.show();
    LOGGER.info("[REQUEST] User: " + "ignite1771" + " buy product: " + "car");
    receipt = applicationServices.loggedInUserPurchase("ignite1771", "car");
    receipt.show();
    LOGGER.info("[REQUEST] User: " + "ignite1771" + " buy product: " + "computer");
    receipt = applicationServices.loggedInUserPurchase("ignite1771", "computer");
    receipt.show();

Salida del programa de cada solicitud:

    Down for maintenance
    Invalid user: abc123
    Out of stock: tv for user = ignite1771 to buy
    Insufficient funds: 1000.0 of user: ignite1771 for buying item: car
    Receipt: 800.0 paid

Diagrama de clases

Special Case

Aplicabilidad

Utilice el patrón Special Case cuando:

• Tienes varios lugares en el sistema que tienen el mismo comportamiento después de una comprobación condicional
  para una instancia de clase en particular, o el mismo comportamiento después de una comprobación nula.
• Devuelve un objeto real que realiza el comportamiento real, en lugar de un objeto nulo que no realiza nada.

Tutorial

• Special Case Tutorial

Créditos

• How to Reduce Cyclomatic Complexity Part 2: Special Case Pattern
• Patterns of Enterprise Application Architecture
• Special Case