#1317 Special Case Pattern (#1624)

* #1317 Add Special Case Pattern

To focus on pattern itself, I implement DB and
maintenance lock by the singleton instance.

* #1317 Add special cases unit tests

Assert the logger output
(ref: https://stackoverflow.com/a/52229629)

* #1317 Add README.md

Add Special Case Pattern README

* #1317 Format: add a new line to end of file

Co-authored-by: Subhrodip Mohanta <subhrodipmohanta@gmail.com>

special-case/src/main/java/com/iluwatar/specialcase/App.java

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+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * <p>The Special Case Pattern is a software design pattern that encapsulates particular cases
+ * into subclasses that provide special behaviors.</p>
+ *
+ * <p>In this example ({@link ReceiptViewModel}) encapsulates all particular cases.</p>
+ */
+public class App {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /**
+   * Program entry point.
+   */
+  public static void main(String[] args) {
+    // 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);
+
+    final 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();
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/ApplicationServices.java

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+package com.iluwatar.specialcase;
+
+public interface ApplicationServices {
+
+  ReceiptViewModel loggedInUserPurchase(String userName, String itemName);
+}

special-case/src/main/java/com/iluwatar/specialcase/ApplicationServicesImpl.java

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+package com.iluwatar.specialcase;
+
+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();
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/Db.java

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+package com.iluwatar.specialcase;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class Db {
+
+  private static Db instance;
+  private Map<String, User> userName2User;
+  private Map<User, Account> user2Account;
+  private Map<String, Product> itemName2Product;
+
+  /**
+   * Get the instance of Db.
+   *
+   * @return singleton instance of Db class
+   */
+  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;
+  }
+
+  /**
+   * Seed a user into Db.
+   *
+   * @param userName of the user
+   * @param amount of the user's account
+   */
+  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);
+  }
+
+  /**
+   * Seed an item into Db.
+   *
+   * @param itemName of the item
+   * @param price of the item
+   */
+  public void seedItem(String itemName, Double price) {
+    Product item = new Product(price);
+    itemName2Product.put(itemName, item);
+  }
+
+  /**
+   * Find a user with the userName.
+   *
+   * @param userName of the user
+   * @return instance of User
+   */
+  public User findUserByUserName(String userName) {
+    if (!userName2User.containsKey(userName)) {
+      return null;
+    }
+    return userName2User.get(userName);
+  }
+
+  /**
+   * Find an account of the user.
+   *
+   * @param user in Db
+   * @return instance of Account of the user
+   */
+  public Account findAccountByUser(User user) {
+    if (!user2Account.containsKey(user)) {
+      return null;
+    }
+    return user2Account.get(user);
+  }
+
+  /**
+   * Find a product with the itemName.
+   *
+   * @param itemName of the item
+   * @return instance of Product
+   */
+  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;
+    }
+
+    /**
+     * Withdraw the price of the item from the account.
+     *
+     * @param price of the item
+     * @return instance of MoneyTransaction
+     */
+    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;
+    }
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/DomainServices.java

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+package com.iluwatar.specialcase;
+
+public interface DomainServices {
+}

special-case/src/main/java/com/iluwatar/specialcase/DomainServicesImpl.java

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+package com.iluwatar.specialcase;
+
+public class DomainServicesImpl implements DomainServices {
+
+  /**
+   * Domain purchase with userName and itemName, with validation for userName.
+   *
+   * @param userName of the user
+   * @param itemName of the item
+   * @return instance of ReceiptViewModel
+   */
+  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);
+  }
+
+  /**
+   * Domain purchase with user, account and itemName,
+   * with validation for whether product is out of stock
+   * and whether user has insufficient funds in the account.
+   *
+   * @param user in Db
+   * @param account in Db
+   * @param itemName of the item
+   * @return instance of ReceiptViewModel
+   */
+  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;
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/DownForMaintenance.java

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+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class DownForMaintenance implements ReceiptViewModel {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(DownForMaintenance.class);
+
+  @Override
+  public void show() {
+    LOGGER.info("Down for maintenance");
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/InsufficientFunds.java

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+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class InsufficientFunds implements ReceiptViewModel {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(InsufficientFunds.class);
+
+  private String userName;
+  private Double amount;
+  private String itemName;
+
+  /**
+   * Constructor of InsufficientFunds.
+   *
+   * @param userName of the user
+   * @param amount of the user's account
+   * @param itemName of the item
+   */
+  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);
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/InvalidUser.java

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+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+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);
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/MaintenanceLock.java

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+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+public class MaintenanceLock {
+
+  private static final Logger LOGGER = LoggerFactory.getLogger(MaintenanceLock.class);
+
+  private static MaintenanceLock instance;
+  private boolean lock = true;
+
+  /**
+   * Get the instance of MaintenanceLock.
+   *
+   * @return singleton instance of MaintenanceLock
+   */
+  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);
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/MoneyTransaction.java

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+package com.iluwatar.specialcase;
+
+public class MoneyTransaction {
+
+  private Double amount;
+  private Double price;
+
+  public MoneyTransaction(Double amount, Double price) {
+    this.amount = amount;
+    this.price = price;
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/OutOfStock.java

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+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+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");
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/ReceiptDto.java

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+package com.iluwatar.specialcase;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+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");
+  }
+}

special-case/src/main/java/com/iluwatar/specialcase/ReceiptViewModel.java

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+package com.iluwatar.specialcase;
+
+public interface ReceiptViewModel {
+
+  void show();
+}