// Private constructor prevents instantiation from other classes
private DataSingletonController(){
this.appInfoMessage ="Tervetuloa Kirjasto-sovellukseen.";
BooksDataAccessObject booksPersistentData = new BooksDataAccessObject();
books = FXCollections.observableArrayList(booksPersistentData.getAll());
AudioRecordsDataAccessObject audioRecordsPersistentData = new AudioRecordsDataAccessObject();
audioRecords = FXCollections.observableArrayList(audioRecordsPersistentData.getAll());
CustomersDataAccessObject customersPersistentData = new CustomersDataAccessObject();