fix: Version number pattern update overwritten #2968 (#2980)

* Fix bug #2968

* Fix bug #2968

version-number/src/main/java/com/iluwatar/versionnumber/BookRepository.java

@@ -24,8 +24,7 @@
  */
 package com.iluwatar.versionnumber;
 
-import java.util.HashMap;
+import java.util.concurrent.ConcurrentHashMap;
-import java.util.Map;
 
 /**
  * This repository represents simplified database.
@@ -34,7 +33,8 @@ import java.util.Map;
  * as much as in real databases.
  */
 public class BookRepository {
-  private final Map<Long, Book> collection = new HashMap<>();
+  private final ConcurrentHashMap<Long, Book> collection = new ConcurrentHashMap<>();
+  private final Object lock = new Object();
 
   /**
    * Adds book to collection.
@@ -57,6 +57,8 @@ public class BookRepository {
       throw new BookNotFoundException("Not found book with id: " + book.getId());
     }
 
+    // used synchronized block to ensure only one thread compares and update the version
+    synchronized (lock) {
       var latestBook = collection.get(book.getId());
       if (book.getVersion() != latestBook.getVersion()) {
         throw new VersionMismatchException(
@@ -71,6 +73,7 @@ public class BookRepository {
       // save book copy to repository
       collection.put(book.getId(), new Book(book));
     }
+  }
 
   /**
    * Returns book representation to the client.