#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/Db.java

@@ -0,0 +1,150 @@
+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;
+    }
+  }
+}