Add base files

src/main/java/com/university/chat/Server/UserManager.java

@@ -0,0 +1,28 @@
+package com.university.chat.Server;
+
+import java.util.concurrent.ConcurrentHashMap;
+
+public class UserManager {
+
+    private final ConcurrentHashMap<String, ClientSession> users = new ConcurrentHashMap<>();
+
+    public boolean addUser(String username, ClientSession session) {
+        return users.putIfAbsent(username, session) == null;
+    }
+
+    public void removeUser(String username) {
+        users.remove(username);
+    }
+
+    public ClientSession getUser(String username) {
+        return users.get(username);
+    }
+
+    public String listUsers() {
+        return users.keySet().toString();
+    }
+
+    public Iterable<ClientSession> getAllSessions() {
+        return users.values();
+    }
+}