根据 JComboBox 中的第一项显示该项

Question

在 GUI 中，我有两个 JComboBox，其中所有的组合框项目都是从两个 tables 检索的，它们是 title 和 time。 In my current code, when the title is selected, displayDate(selected); will get called since I have implemented addActionListener in Select comboBox.

但这不是我想要的。运行此文件时，我希望它立即根据 Select JcomboBox 中的第一项显示日期。 When the Select comboBox item changed, only the date changed.正确的写作方式是什么？

  public class BuyTicket {  
    static JFrame frame;
    JLabel title,lblMainDate,selectMovie,dateOfShow;
    JComboBox Select,Date;

    public JPanel createContentPane() throws IOException
    {

        title = new JLabel("CYBER CINEMA");
        Select = new JComboBox();
        Select.setLocation(115,90);
        Select.setSize(175, 20);
        try {   
            DatabaseConnection db=new DatabaseConnection();
            Connection connect=db.getConnection();
            String sql="Select title FROM movie";
            PreparedStatement ps=connect.prepareStatement(sql);
            ResultSet rs = ps.executeQuery();
            while (rs.next()) {
            String name = rs.getString("title");
            Select.addItem(name);         
        }

    } catch (Exception e) {
        System.out.println("null");
    }


        Select.addActionListener(new ActionListener()
        {
    public void actionPerformed(ActionEvent event)
        {
         JComboBox comboBox=(JComboBox) event.getSource();
         Object selected = Select.getSelectedItem();
         displayDate(selected);
        }

    private void displayDate(Object selected) {
        // TODO Auto-generated method stub
          try { 
                Date.removeAllItems();
                DatabaseConnection db=new DatabaseConnection();
                Connection connect=db.getConnection();
                String sql="Select date FROM movie WHERE title = ?";
                PreparedStatement ps=connect.prepareStatement(sql);
                ps.setObject(1, selected);
                ResultSet rs = ps.executeQuery();
                while (rs.next()) {
                String date1 = rs.getString("date");
                DefaultComboBoxModel model = (DefaultComboBoxModel)Date.getModel();
                if (model.getIndexOf(date1) == -1)
                {
                    Date.addItem(date1);
                }

            }

        } catch (Exception e) {
            System.out.println("null");
        }


    }
        });
    }

已编辑

public class BuyTicket {

    static JFrame frame;
    JLabel title,lblMainDate,selectMovie,dateOfShow;
    JComboBox Select,Date;

    public JPanel createContentPane() throws IOException
    {
        Select = new JComboBox();
        Select.setLocation(115,90);
        Select.setSize(175, 20);
        try {   
            DatabaseConnection db=new DatabaseConnection();
            Connection connect=db.getConnection();
            String sql="Select title FROM movie";
            PreparedStatement ps=connect.prepareStatement(sql);
            ResultSet rs = ps.executeQuery();
            while (rs.next()) {
            String name = rs.getString("title");
            Select.addItem(name);         
        }

    } catch (Exception e) {
        e.printStackTrace();
    }

        Select.addActionListener(new ActionListener()
        {
    public void actionPerformed(ActionEvent event)
        {
         JComboBox comboBox=(JComboBox) event.getSource();
         Object selected = Select.getSelectedItem();
         displayDate(selected);
        }
        });

        String getMovie = (String)Select.getSelectedItem(); // New code added, directly display the Date JComboBox items 
        System.out.println(getMovie);
        displayDate(getMovie);
        Date = new JComboBox();
        Date.setLocation(115,140);
        Date.setSize(175, 20);
    }

    private void displayDate(Object selected) {
        // TODO Auto-generated method stub
          try { 
                Date.removeAllItems();
                DatabaseConnection db=new DatabaseConnection();
                Connection connect=db.getConnection();
                String sql="Select date FROM movie WHERE title = ?";
                PreparedStatement ps=connect.prepareStatement(sql);
                ps.setObject(1, selected);
                ResultSet rs = ps.executeQuery();
                while (rs.next()) {
                String date1 = rs.getString("date");
                DefaultComboBoxModel model = (DefaultComboBoxModel)Date.getModel();
                if (model.getIndexOf(date1) == -1)
                {
                    Date.addItem(date1);
                }

            }

        } catch (Exception e) {
            e.printStackTrace();
        }


    }

}

我添加了一些代码，但日期仍然无法直接显示，并且出现nullpointer 错误。

最新输出

Angry Bird
java.lang.NullPointerException
    at gui.BuyTicket.displayDate(BuyTicket.java:131)
    at gui.BuyTicket.createContentPane(BuyTicket.java:87)
    at gui.BuyTicket.createAndShowGUI(BuyTicket.java:115)
    at gui.HomePage$2.mouseClicked(HomePage.java:151)
    at java.awt.Component.processMouseEvent(Unknown Source)

Answer 1

将您的selectDate(...) 设为BuyTicket 类的方法，将其移出ActionListener 的范围。然后在完成第一次数据库搜索并填充第一个 JComboBox 后，直接调用此方法，传入第一个 JComboBox 中的第一项。

即，改变这个：

public class BuyTicket {
    //...

    public JPanel createContentPane() throws IOException {
        // ...
        Select.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent event) {
                // ...
            }

            // within addActionListener scope
            private void displayDate(Object selected) {
                // ...
            }
        });
        // ...
    }
}

到这里：

public class BuyTicket {
    //...

    public JPanel createContentPane() throws IOException {
        // ...
        Select.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent event) {
                // ...
            }

        });
        // ...
    }

    // within class scope
    private void displayDate(Object selected) {
        // ...
    }
} 

---

作为附带建议，您将希望学习和使用Java naming conventions。变量名应全部以小写字母开头，而类名应以大写字母开头。学习这一点并遵循这一点将使我们能够更好地理解您的代码，并让您更好地理解其他人的代码。

---

建议 2：您将希望在后台线程中执行所有数据库搜索，例如 SwingWorker。您将希望使用回调机制（例如 PropertyChangelistener）在工作人员完成时通知您，以便您可以提取和显示它收集的数据。例如：

• Get and send messages with Java Threads
• Dialog isn't responsive while SwingWorker works in the background
• Repeat SwingWorker

更多内容请查看教程——Lesson: Concurrency in Swing

---

建议 3：您真的不想拥有这样的代码：

} catch (Exception e) {
    System.out.println("null");
}

至少打印出堆栈跟踪（例如，e.printStacktrace();），这样您就会知道发生了哪些异常（如果有）。此外，您应该捕获特定异常，而不是过于宽泛的Exception。

---

关于：

java.lang.NullPointerException
    at gui.BuyTicket.displayDate(BuyTicket.java:131)
    at gui.BuyTicket.createContentPane(BuyTicket.java:87)
    at gui.BuyTicket.createAndShowGUI(BuyTicket.java:115)
    at gui.HomePage$2.mouseClicked(HomePage.java:151)
    at java.awt.Component.processMouseEvent(Unknown Source)

您现在遇到了一个全新且独特的问题。请为我们指出哪一行是BuyTicket.java:131，因为该行上有一个为空的变量，并且您尝试使用它，就好像它引用了一个对象一样。调试此问题的关键是批判性地查看该行，查看哪个变量为空或使用调试器或 println 来帮助确定哪个为空，然后修复您的代码，使其不再为空。见：What is a NullPointerException, and how do I fix it?

---

线BuyTicket.java:131 是Date.removeAllItems();。错误似乎来自 displayDate(getMovie);，我刚刚添加的新代码

所以 Date JComboBox 为空，如果您查看代码，很明显为什么，以及如何修复它——在构造函数中创建它，而不是在 ActionListener 中。