使用removeAllItems（）后，JComboBox ActionListener无法正常工作

I have two JComboBoxes; one removes all items in the other if it's already populated and then adds a new set of items, and the second fires an event that gets information from a database using the selected item. The problem occurs after the first combo box does its thing removing the items and adding new ones; when I select any of the items in the second JComboBox, the event that fires doesn't happen anymore.

我有两个JComboBox;如果已经填充了另一项，则删除另一项中的所有项目，然后添加一组新项目，第二项将使用所选项目触发从数据库获取信息的事件。在第一个组合框删除项目并添加新项目后，会出现问题;当我选择第二个JComboBox中的任何项目时，不再发生触发的事件。

Below I have provided snippets of my code:

下面我提供了我的代码片段：

The first combo box

第一个组合框

    cmbIDs.addActionListener(new ActionListener()
    {
        public void actionPerformed(ActionEvent e)
        {
            selection = (String)cmbIDs.getSelectedItem();
            if (!(selection.equals("Select an username")))//current selection in combobox is stored as string
            {
                comboActivate(selection);
                if (!unitC.getText().equals("")){
                    unitC.setText("");
                }
                if (!lecturer.getText().equals("")){
                    lecturer.setText("");
                }

                if (!(courseD.getText().equals("Not Enrolled"))){    
                    populateUnits(selection);
                }

            }
            else{
                JOptionPane.showMessageDialog(null,"Please select a Surname.");
            }
        }
    });

Removing the items inside populateUnits(String selectionID):

删除populateUnits（String selectionID）中的项：

    try 
    {
        units.removeAllItems();
        units.addItem("Select a Unit");
    }
    catch (NullPointerException npe)
    {
        units.addItem("Select a Unit"); 
    }

After this instructions are sent through a client to a server where a db is queried and the server replies with information which is then added to the second JComboBox. I assure you also that the items are added to the JComboBox after removeAllItems() is used.

在此指令通过客户端发送到查询数据库的服务器之后，服务器回复信息，然后将信息添加到第二个JComboBox。我还向您保证，在使用removeAllItems（）之后，项目将添加到JComboBox中。

The second jComboBox:

第二个jComboBox：

units.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent ue)
    {
        uSelect = (String)units.getSelectedItem();
        if (!(uSelect.equals("Select a Unit")))//current selection in combobox is stored as string
        {
            System.out.println(uSelect);
            unitActivate(uSelect);
        }
        else
        {
            JOptionPane.showMessageDialog(null,"Please select a Unit.");
        }
    }
});

1 个解决方案

#1

It looks like your code never gets a fresh set of items from the database, so the user can never select anything other than "Select an Unit", which your second code block ignores.

看起来您的代码永远不会从数据库中获取一组新的项目，因此用户永远不会选择“选择一个单元”以外的任何内容，这是您的第二个代码块忽略的。