private void saveBankUsersToJson() {
        try (FileWriter writer = new FileWriter("bank_users.json")) {
            gson.toJson(bankUsers, writer);

        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private void loadBankUsersFromJsontoHashMap() {
        try (Reader reader = new FileReader("bank_users.json")) {
            bankUsers = gson.fromJson(reader, new TypeToken<HashMap<Integer, Object>>() {
            }.getType());
        } catch (IOException e) {
            e.printStackTrace();
        }

    }