ExampleCode

/**
 *
 * @author Pudding
 */
public class Test {

    static class BeanUsers {

        private String userID;

        public BeanUsers() {
            this("");
        }

        public BeanUsers(String userID) {
            this.userID = userID;
        }

        public String getUserID() {
            return userID;
        }
    }

    public static class FileHandler {

        private FileHandler() {
        } //Konstruktor
        private ArrayList<BeanUsers> allUser = new ArrayList<>();
        private static FileHandler instance = null;
        private BeanUsers currentUser = new BeanUsers();

        public static synchronized FileHandler getInstance() {
            if (instance == null) {
                instance = new FileHandler();
            }
            return instance;
        }

        public int addNewUser(BeanUsers neuer) {
            this.allUser.add(neuer); //hier soll es eigentlich nur ANGEHANGEN werden
            return this.allUser.indexOf(neuer); //gibt index, des neuen Elementes zurück
        }

        public ArrayList<BeanUsers> getAllUser() {
            return allUser;
        }

    }

    public static void main(String[] args) {
        FileHandler handler = FileHandler.getInstance();
        System.out.println(handler.addNewUser(new BeanUsers("Pudding")));
        System.out.println(handler.addNewUser(new BeanUsers("Pudding2")));
        for (BeanUsers user : handler.getAllUser()) {
            System.out.println(user.getUserID());
        }
    }

}