#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/DomainServicesImpl.java

@@ -0,0 +1,46 @@
+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;
+  }
+}