Java中的侦听器有问题。我有一个下拉框，没有像我需要的那样传递用户选择

11 月，3 周 Questions & Answers 75

我有一个本周三要上的课的项目，我有一些问题：我有两个if-else语句控制两个不同下拉菜单中显示的值。对我来说，似乎我没有得到上一个下拉列表的信息（有两个下拉列表会影响下一个下拉列表中显示的值）

以下是我迄今为止的代码：

///Occupancy///
JLabel label2 = new JLabel("Please Select the Number of Occupants");
JComboBox occupancy_list = new JComboBox(occupancy_string);
occupancy_list.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent event)//Listener for Occupancy Drop Down
      {
          JComboBox cb = (JComboBox)event.getSource(); //grab the user selection
          String selection = (String) cb.getSelectedItem();
          int number = Integer.parseInt((String) selection);
      }
});
String selection = (String) occupancy_list.getSelectedItem();
int number = Integer.parseInt((String) selection);
if(number>2)
{
  style=(style2);
}
else
{
  style=(style1);
}

///Room Type///
JLabel label3 = new JLabel("Please Select a Room Style");

//Creates RoomStyle Drop Down
JComboBox room_type = new JComboBox(style);
roomtype_string=(String) room_type.getSelectedItem();
room_type.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent event)//Listener for Room Style Drop Down
      {

          JComboBox cb = (JComboBox)event.getSource(); //grab the user selection
          String selection = (String) cb.getSelectedItem();

      }
});
String selection2 = (String) room_type.getSelectedItem();
if(selection2.equals("Cabin"))
{
  room_number=(cabin_string);
}
else
{
  room_number=(suite_string);
}

///Room Selection///
JLabel label4 = new JLabel("Please Select a Room");

//Creates RoomNumber Drop Down
JComboBox room_list = new JComboBox(room_number);
roomselected = (String) room_list.getSelectedItem();
room_list.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent event)//Listener for Occupancy Drop Down
      {
          JComboBox cb = (JComboBox)event.getSource(); //grab the user selection
          String selection = (String) cb.getSelectedItem();
      }
});
String selection3 = (String) room_list.getSelectedItem();

//Dining
JLabel label5 = new JLabel("Please Select a Dining Time");
JComboBox dining_list = new JComboBox(dining_string);
dining_list.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent event)//Listener for Occupancy Drop Down
      {
          JComboBox cb = (JComboBox)event.getSource(); //grab the user selection
          String selection = (String) cb.getSelectedItem();
      }
});
String selection4 = (String) dining_list.getSelectedItem();

注意：我已经重写了我的代码，仍然没有骰子： `//创建包含占用率、房间类型、房间和就餐时间请求的子面板2 JPanel subpanel=新的JPanel（）

    ///Occupancy///
    JLabel label2 = new JLabel("Please Select the Number of Occupants");
    JComboBox occupancy_list = new JComboBox(occupancy_string);
    Occupancy_Listener occupancy = new Occupancy_Listener();
    occupancy_list.addActionListener(occupancy);
    //updateStyle(occupancy_string[occupancy_list.getSelectedIndex()]);

    ///Room Type///
    JLabel label3 = new JLabel("Please Select a Room Style");

    //Creates RoomStyle Drop Down
    JComboBox room_type = new JComboBox(style);
    Style_Listener styleListen = new Style_Listener();
    room_type.addActionListener(styleListen);
    //updateNumber(style[room_type.getSelectedIndex()]);

    ///Room Selection///
    JLabel label4 = new JLabel("Please Select a Room");

    //Creates RoomNumber Drop Down
    JComboBox room_list = new JComboBox(room_number);
    Room_Listener room = new Room_Listener();
    room_list.addActionListener(room);
    //updateRoom(room_number[room_list.getSelectedIndex()]);

    //Dining
    JLabel label5 = new JLabel("Please Select a Dining Time");
    JComboBox dining_list = new JComboBox(dining_string);
    Din_Listener dining = new Din_Listener();
    dining_list.addActionListener(dining);
    //updateDin(dining_string[dining_list.getSelectedIndex()]);
    ...
    ...
    ...
    ...
    ...
  }

  private class Occupancy_Listener implements ActionListener
  {
      public void actionPerformed(ActionEvent event)//Listener for Occupancy Drop Down
      {
          JComboBox cb = (JComboBox)event.getSource(); //grab the user selection
          String selection = (String) cb.getSelectedItem();
          System.out.println(selection);
          System.out.println(style[0]);
          System.out.println(room_number[0]);
          updateStyle(selection);
      }

  }

  private class Style_Listener implements ActionListener
  {
      public void actionPerformed(ActionEvent event)//Listener for Occupancy Drop Down
      {


          JComboBox cb = (JComboBox)event.getSource(); //grab the user selection
          String selection = (String) cb.getSelectedItem();
          updateNumber(selection);

      }
  }
  private class Room_Listener implements ActionListener
  {
      public void actionPerformed(ActionEvent event)//Listener for Occupancy Drop Down
      {
          JComboBox cb = (JComboBox)event.getSource(); //grab the user selection
          String selection = (String) cb.getSelectedItem();
          updateRoom(selection);
      }
  }
  private class Din_Listener implements ActionListener
  {
      public void actionPerformed(ActionEvent event)//Listener for Occupancy Drop Down
      {
          JComboBox cb = (JComboBox)event.getSource(); //grab the user selection
          String selection = (String) cb.getSelectedItem();
          updateDin(selection);
      }
  }
  protected void updateStyle(String pick) 
  {
      String[] style1 ={"Cabin", "Suite"};
      String[] style2 ={"Suite"};
      //ocu_string=pick;
      number = Integer.parseInt(pick);
      if(number>2)
      {
        style=style2;
      }
      else
      {
        style=style1;
      }
  }
  protected void updateNumber(String pick) 
  {
      String[] cabin_string = {"11-1","11-2","11-3","11-4","11-5","11-6","11-7","11-8","11-9","11-10"};
      String[] suite_string = {"11-S1","11-S2"};
      type=pick;
      if(type.equals("cabin"))
      {
          room_number=cabin_string;
      }
      else
      {
          room_number=suite_string;
      }
  }
  protected void updateRoom(String pick) 
  {
      room_num=pick;
  }
  protected void updateDin(String pick) 
  {
      din_time=pick;
  }
  //public String getPopulation()
  {
      //return ocu_string;
  }

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共 (2) 个答案

# 1 楼答案
匿名类（或本地类）中的代码不会与周围的代码同时执行，这可能会让您感到困惑

所以在一段代码中，它看起来像：
```
A a = new A();
a.setSomething( new B() {
    public void bMethod() {
        // Run stuff
    }
} );
C c = a.getSomething();
```
“运行东西”部分现在不运行。创建了一个对象，其中包含一个稍后可以在其中使用的方法，然后该对象被传递到。该方法将不会运行，直到有人专门调用它。当您到达getSomething()时，“Run stuff”部分还没有运行

因此，在您的代码中：
```
JComboBox occupancy_list = new JComboBox(occupancy_string);
occupancy_list.addActionListener(new ActionListener()
{
    public void actionPerformed(ActionEvent event)//Listener for Occupancy Drop Down
      {
          JComboBox cb = (JComboBox)event.getSource(); //grab the user selection
          String selection = (String) cb.getSelectedItem();
          int number = Integer.parseInt((String) selection);
      }
});
String selection = (String) occupancy_list.getSelectedItem();
```
创建占用列表，为其创建操作侦听器，并将该操作侦听器存储在占用列表中。但是actionPerformed()方法中编写的内容在GUI显示、用户选择某个内容并触发事件之前都不会运行

GUI编程不同于控制台编程，在控制台编程中，您调用“scan（）”，然后程序停止并等待输入内容。首先准备GUI，当显示GUI时，会根据用户的操作调用各种侦听器

因此，在将列表添加到GUI之前，您的代码直接跳转到occupancy_list.getSelectedItem()，当然，没有选择任何内容，也没有显示任何内容，因此其他下拉列表是在用户看到GUI之前创建的

对于代码中的所有操作侦听器都是如此

编写动作监听器的正确方法是考虑GUI已经运行时的条件。您的操作侦听器仅在将被丢弃的局部变量中设置值。相反，当用户在操作侦听器中进行选择时，它们应该做您想做的事情

这意味着此组合框的操作侦听器必须创建其他列表并将其添加到GUI中，重新验证并重新绘制。或者，它可以简单地修改已经创建的其他列表，这取决于您认为最好的列表。但是他们必须访问这些列表，因此必须将这些列表定义为周围类的字段，并在操作侦听器中访问。请注意，您还可以定义一个执行此操作的方法，并从操作侦听器内部调用它

我的建议是，首先创建一个简单的GUI，只包含第一个组合框，并根据选择更改一些简单的内容。例如，您可以遵循Oracle Tutorial。了解如何编写更改简单内容的操作侦听器后，展开程序以添加其他列表，并从操作侦听器中操作它们

# 2 楼答案

这可能就是你的问题所在。从第一个组合框中选择一个项目，并将其填充到第二个组合框中：

import java.awt.*;
import java.awt.event.*;
import java.util.*;
import javax.swing.*;

public class ComboBoxTwo extends JPanel implements ActionListener
{
    private JComboBox<String> mainComboBox;
    private JComboBox<String> subComboBox;
    private Hashtable<String, String[]> subItems = new Hashtable<String, String[]>();

    public ComboBoxTwo()
    {
        String[] items = { "Select Item", "Color", "Shape", "Fruit" };
        mainComboBox = new JComboBox<String>( items );
        mainComboBox.addActionListener( this );

        //  prevent action events from being fired when the up/down arrow keys are used
        mainComboBox.putClientProperty("JComboBox.isTableCellEditor", Boolean.TRUE);
        add( mainComboBox );

        //  Create sub combo box with multiple models

        subComboBox = new JComboBox<String>();
        subComboBox.setPrototypeDisplayValue("XXXXXXXXXX"); // JDK1.4
        add( subComboBox );

        String[] subItems1 = { "Select Color", "Red", "Blue", "Green" };
        subItems.put(items[1], subItems1);

        String[] subItems2 = { "Select Shape", "Circle", "Square", "Triangle" };
        subItems.put(items[2], subItems2);

        String[] subItems3 = { "Select Fruit", "Apple", "Orange", "Banana" };
        subItems.put(items[3], subItems3);
    }

    public void actionPerformed(ActionEvent e)
    {
        String item = (String)mainComboBox.getSelectedItem();
        Object o = subItems.get( item );

        if (o == null)
        {
            subComboBox.setModel( new DefaultComboBoxModel() );
        }
        else
        {
            subComboBox.setModel( new DefaultComboBoxModel( (String[])o ) );
        }
    }

    private static void createAndShowUI()
    {
        JFrame frame = new JFrame("SSCCE");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add( new ComboBoxTwo() );
        frame.setLocationByPlatform( true );
        frame.pack();
        frame.setVisible( true );
    }

    public static void main(String[] args)
    {
        EventQueue.invokeLater(new Runnable()
        {
            public void run()
            {
                createAndShowUI();
            }
        });
    }
}

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Java中的侦听器有问题。我有一个下拉框，没有像我需要的那样传递用户选择

共 (2) 个答案

# 1 楼答案

# 2 楼答案