JAVAlang.NullPointerException？将get方法指定给arraylist时

Question

您好，我正在尝试显示从另一个类添加到“表”类的客户订单

public class PaymentScreenController {


    public ArrayList<Customer> allcustomers; // Defining allcustomers Arraylist




    public Table tbl; // Creating instance of Table class


    @FXML
    public void initialize() {

        allcustomers = tbl.getCustomers(); // Error seems to come from here when i assign allcustomers to the getcustomers method of table
        System.out.println(tbl.getCustomers());

        System.out.println("test");
        for (Customer customer : allcustomers) {

            System.out.println(customer.getCustomerorder());
        }


    }
}

任何帮助都将不胜感激，谢谢

**更多代码*

public void submitorder(ActionEvent actionEvent) throws IOException {
        //create a new UNIQUE customer to store that customers menu items
        Customer cust = new Customer();
        //add items to customer
        cust.getCustomerorder().addAll(order);
        // Adds the customer order to the table class
        tbl.getCustomers().add(cust);
        System.out.println(tbl.getCustomers());
        //clear menu choices screen
        lvOrder.getItems().clear();

        // Once added the desired number of customers move to next screen
        //extra customer was being added as a customer was put into the ArrayLust after the check happened.
        if (tbl.getCustomers().size() == noOfDiners) {
            Window mainWindow = btnSubmitOrder.getScene().getWindow();
            Parent newRoot = FXMLLoader.load(getClass().getResource("payment_screen.fxml"));
            mainWindow.getScene().setRoot(newRoot);
        }

    }

因此，客户向他们的订单中添加内容，然后客户被添加到表类中

Answer 1

如果NPE在你的评论中提到的那句话中提出，这意味着， 当调用方法initialize()时，变量tbl不会被实例化并保持null值

当你调用tbl.getCustomers()时，它会导致NullPointerException，因为tbl在这里是空的

我没有看到任何代码，创建表的对象并将其分配给变量。也许它发生在这个类之外的某个地方，因为你的变量有public修饰符，那么请确保这个代码调用的时间早于initialize方法

你需要创建一个Table的对象，比如：

public Table tbl = new Table(..);

要获得更具体的答案，请在这里分享你的堆栈跟踪

Answer 2

public Table tbl; // Creating instance of Table class

在这里，您只是声明了表，而不是创建它的实例

要做到这一点，你必须做tbl = new Table(insert arguments here);

这就是为什么你有一个空指针