change_user_details method username change logic fixed

src/main/java/de/hs_mannheim/domain/System.java

@@ -180,9 +180,11 @@ public class System {
         if(username.equals("")||hometown.equals("")||zip.equals(""))
             return false;
         
+        if(!current_user.getUsername().equals(username)){
             for(User user: this.all_user)
                 if(user.getUsername().equals(username))
                     return false;
+        }
         
         ArrayList<String> mem = search(zip);
         boolean bool = false;