#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/InvalidUser.java

@@ -0,0 +1,20 @@
+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);
+  }
+}