User will be deleted when connection lost or disconnected for one reason.

src/server/Server.java

@@ -46,4 +46,8 @@ public class Server {
 				u.sendMessage(message);
 		}
 	}
+	
+	public void deleteUser(User user){
+		users.remove(user);
+	}
 }

src/server/User.java

@@ -35,7 +35,8 @@ public class User {
 						//TODO: receive message from user according our protocol
 						server.addMessage(new Message("messageINPUT", User.this));
 					} catch (IOException e) {
-						e.printStackTrace();
+						System.out.println("User disconnected: " + username + "from: " + client.getLocalSocketAddress());
+						server.deleteUser(User.this);
 					}
 				}					
 			}