合并Java中的2个列表

Question

如何在Java中合并2个列表？

我想从数据库中检索数据行，然后对这些数据进行一些编辑：添加，删除，编辑记录。但是当我添加新记录并重新加载我的页面时，数据库中的所有记录都是重复的！？

出了什么问题？我的缺陷在哪里？

提前致谢！

我的名单：

private List<Order> orderList = new ArrayList<>(getAllUsers());

public List<Order> getOrderList() {
    return orderList;
}

public List<Order> getAllUsers() {
    try {
        orderList = new ArrayList<>();
        Connection connection = databaseConnection.getConnection();
        SQL = "SELECT * FROM Registration";
        prepStatement = connection.prepareStatement(SQL);
        resultSet = prepStatement.executeQuery();
        boolean foundResult = false;
        while (resultSet.next() == true) {
            orders = new Order(id, orderNo, productName, price, qty);
            orders.setId(resultSet.getInt("id"));
            orders.setOrderNo(resultSet.getString("orderNo"));
            orders.setProductName(resultSet.getString("productName"));
            orders.setPrice(resultSet.getBigDecimal("price"));
            orders.setQty(resultSet.getInt("qty"));
            orderList.add(orders);
            foundResult = true;
        }
        resultSet.close();
        prepStatement.close();
        if (foundResult) {

            return orderList;

        } else {
            return null; // no entries found.
        }
    } catch (Exception eX) {
        // TODO: handle exception
        System.out.println("Error in getMeineList()--> " + eX.getMessage());
        return (null);
    }

}

public String addNew() {
    orders = new Order();
    orders.setEditable(true);
    orderList.add(orders);
    return "";
}

public String edit(Order ord) {
    ord.setEditable(true);
    return null;
}

public void insert() {
    // int q= 0;
    System.out.println("Inserting...!");
    try {
        connection = databaseConnection.getConnection();
        statement = connection.createStatement();
        SQL = "INSERT INTO Registration (orderNo,productName,price,qty) VALUES (?,?,?,?)";
        prepStatement = connection.prepareStatement(SQL);
        prepStatement.setString(1, orderNo);
        prepStatement.setString(2, productName);
        prepStatement.setBigDecimal(3, price);
        prepStatement.setInt(4, qty);
        prepStatement.executeUpdate();
        System.out.println("User added Successfully!");
        FacesMessage message = new FacesMessage("New User added!");
        FacesContext.getCurrentInstance().addMessage("adminForm: saveButton", message);
        statement.close();
        prepStatement.close();
        connection.close();
    } catch (SQLException SQLex) {
        // TODO: handle exception
        SQLex.printStackTrace();
    } catch (Exception IOex) {
        // TODO: handle exception
        IOex.printStackTrace();
    }
}