public static void usersUpdate( String attributeToBeChanged, String newAttributeValue, String whereKey ,String whereValue) throws Exception{


        Method getter=getDeclaredMethodIgnoreCase(User.class,"get"+whereKey);

        HashMap<String,User> refToUsersCache= usersCache.getUsersCache(); // The objects are stored in this map as <username,User Object>
        User requiredUser = null;

        for(String user: refToUsersCache.keySet()) { // loop through the hashmap and call its getter to check its "ID" or any attribute
            if (getter.invoke(refToUsersCache.get(user)).equals(whereValue))  // getID() == "5" (in our example)
                requiredUser = refToUsersCache.get(user); // this is the required object
        }

        Method setter=getDeclaredMethodIgnoreCase(User.class,"set"+attributeToBeChanged);
        setter.invoke(requiredUser,newAttributeValue); // change the username = "newValue"

);