/**
     * This methods creates a User File Data Access Object.
     * @param filename the name of the JSON file to be read and write to
     * @param objectMapper is used to convert a User JSON object to/from a User Java object for serialization and deserialization
     * @throws IOException throws when there is an issue with the JSON file
     */
    public UserFileDAO(@Value("${users.file}") String fileName, ObjectMapper objectMapper) throws IOException{
        this.fileName = fileName;
        this.objectMapper = objectMapper;
        load();
    }

    /**
     * This method loads a User object from the JSON file into the Tree map.
     * @return returns map of User objects
     * @throws IOException throws when there is an issue with the JSON file
     */
    private Map<String, User> load() throws IOException{
        users = new TreeMap<>();

        User[] userList = objectMapper.readValue(new FileReader(fileName), User[].class);

        for(User user: userList){
            users.put(user.getUsername(), user);
        }
        return users;
    }

    /**
     * This method is used to save a User objects from the tree map into the JSON file.
     * @return returns true if the User objects were successfully written into the JSON file.
     * @throws IOException throws when there is an issue with the JSON file
     */
    private boolean save() throws IOException{
        User[] userList = getUsersList();
        objectMapper.writeValue(new File(fileName), userList);
        return true;
    }

    /**
     * This method is used to make a list of user object from the Tree Map for any User object that matches the keyword.
     * If the keyword is null, all the users in the Tree Map will be in the list.
     * @param keyword the string to be looked for in the Tree Map that could be added to the User object list
     * @return a populated or empty list of User objects
     */
    private User[] getUsersList(String keyword){
        ArrayList<User> userList = new ArrayList<>();

        for(User user: users.values()){
            if(keyword == null || user.getUsername().contains(keyword)){
                userList.add(user);
            }
        }

        User[] userListNew = new User[userList.size()];
        userList.toArray(userListNew);
        return userListNew;
    }

    /**
     * This method is used to generate a list of User objects from the tree map.
     * @return a populated or empty list of User objects
     */
    private User[] getUsersList(){
        return getUsersList(null);
    }