我正在尝试从ActionListener更改JComboBox的值，并且没有运气。
我有一个JComboBox，当更改时更改了CardLayout（切换到另一个GUI）。
我有一个菜单栏，其中包含用户选项，我希望自动更改JComboBox selectedItem。从本质上讲，我的最终目的是让菜单栏可以更改CardLayout（但我无法接近使用ActionListener更改卡片布局）。
看起来代码无法从内部侦听器类中找到JComboBox。
任何帮助，包括我参考的参考资料，都将不胜感激。
这是我目前的代码（仍在开发中）：
package inventorytrackingsystem;
import java.io.*;
import java.net.*;
import java.util.*;
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.rmi.*;
public class InventoryTrackingSystem implements ItemListener {
JPanel mainPanel;
JFrame frame;
JPanel cards; //a panel that uses CardLayout
final static String MAINPANEL = "Main";
final static String CHECKITEMSPANEL = "Check Items";
final static String NEWITEMPANEL = "New Item";
final static String CHECKOUTITEMPANEL = "Check Out Item";
final static String ITEMINFOPANEL = "Item Information";
JPanel comboBoxPane;
private JComboBox cb;
static String comboBoxItems[] = {MAINPANEL,CHECKITEMSPANEL,NEWITEMPANEL,CHECKOUTITEMPANEL,ITEMINFOPANEL};
public static void main(String[] args) {
final InventoryTrackingSystem ITS = new InventoryTrackingSystem();
/* Use an appropriate Look and Feel */
try {
UIManager.setLookAndFeel("javax.swing.plaf.metal.MetalLookAndFeel");
} catch (UnsupportedLookAndFeelException ex) {
ex.printStackTrace();
} catch (IllegalAccessException ex) {
ex.printStackTrace();
} catch (InstantiationException ex) {
ex.printStackTrace();
} catch (ClassNotFoundException ex) {
ex.printStackTrace();
}
/* Turn off metal's use of bold fonts */
UIManager.put("swing.boldMetal", Boolean.FALSE);
//Schedule a job for the event dispatch thread:
//creating and showing this application's GUI.
// javax.swing.SwingUtilities.invokeLater(new Runnable() {
// public void run() {
ITS.createAndShowGUI();
// }
// });
}
public void addComponentToPane(Container pane) {
//Put the JComboBox in a JPanel to get a nicer look.
comboBoxPane = new JPanel(); //use FlowLayout
cb = new JComboBox(comboBoxItems);
cb.setEditable(false);
cb.addItemListener(this);
comboBoxPane.add(cb);
//Create the "cards".
JPanel main = new guiBuilder().buildGui("main");
JPanel checkItems = new guiBuilder().buildGui("checkItems");
JPanel newItems = new guiBuilder().buildGui("newItems");
JPanel checkOutItems = new guiBuilder().buildGui("checkOutItems");
JPanel itemInfo = new guiBuilder().buildGui("itemInfo");
//Create the panel that contains the "cards".
cards = new JPanel(new CardLayout());
cards.add(main, MAINPANEL);
cards.add(checkItems, CHECKITEMSPANEL);
cards.add(newItems, NEWITEMPANEL);
cards.add(checkOutItems, CHECKOUTITEMPANEL);
cards.add(itemInfo, ITEMINFOPANEL);
pane.add(comboBoxPane, BorderLayout.PAGE_START);
pane.add(cards, BorderLayout.CENTER);
}
public void itemStateChanged(ItemEvent evt) {
CardLayout cl = (CardLayout)(cards.getLayout());
cl.show(cards, (String)evt.getItem());
}
/**
* Create the GUI and show it. For thread safety,
* this method should be invoked from the
* event dispatch thread.
*/
private void createAndShowGUI() {
//Create and set up the window.
frame = new JFrame("Inventory Tracking System");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setLayout(new BorderLayout());
Menu m1 = new Menu("Options");
MenuItem mi1_0 = new MenuItem("Main Page");
mi1_0.setActionCommand("main");
mi1_0.addActionListener(new menuListener());
MenuItem mi1_1 = new MenuItem("Check Item");
mi1_1.setActionCommand("checkItem");
mi1_1.addActionListener(new menuListener());
MenuItem mi1_2 = new MenuItem("Add New Item");
mi1_2.setActionCommand("addItem");
mi1_2.addActionListener(new menuListener());
MenuItem mi1_3 = new MenuItem("List All Items");
mi1_3.setActionCommand("listAllItems");
mi1_3.addActionListener(new menuListener());
MenuItem mi1_4 = new MenuItem("Exit");
mi1_4.setActionCommand("exit");
mi1_4.addActionListener(new menuListener());
Menu m2 = new Menu("Help");
MenuItem mi2_0 = new MenuItem("About");
mi2_0.setActionCommand("about");
mi2_0.addActionListener(new menuListener());
m1.add(mi1_0);
m1.add(mi1_1);
m1.add(mi1_2);
m1.add(mi1_3);
m1.add(mi1_4);
m2.add(mi2_0);
MenuBar mb = new MenuBar();
frame.setMenuBar(mb);
mb.add(m1);
mb.add(m2);
//Create and set up the content pane.
InventoryTrackingSystem setGUI = new InventoryTrackingSystem();
setGUI.addComponentToPane(frame.getContentPane());
//Display the window.
frame.pack();
frame.setVisible(true);
frame.setSize(500, 500);
frame.addWindowListener(new WindowAdapter(){
public void windowClosing(WindowEvent we){
System.exit(0);
}
public void windowClosed(WindowEvent we){
System.exit(0);
}
});
}
class menuListener implements ActionListener{
public void actionPerformed(ActionEvent ev){
String thisAction=ev.getActionCommand();
if(thisAction.equals("main")){
cb.setSelectedItem(MAINPANEL);
}else if(thisAction.equals("checkItem")){
//change GUI
}else if(thisAction.equals("addItem")){
//change GUI
}else if(thisAction.equals("listAllItems")){
//change GUI
}else if(thisAction.equals("exit")){
System.exit(0);
}else if(thisAction.equals("about")){
JOptionPane.showMessageDialog(frame, "About This App");
}
}
}
}
我尝试使用菜单时遇到的错误如下：
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at inventorytrackingsystem.InventoryTrackingSystem$menuListener.actionPerformed(InventoryTrackingSystem.java:161)
at java.awt.MenuItem.processActionEvent(MenuItem.java:627)
at java.awt.MenuItem.processEvent(MenuItem.java:586)
at java.awt.MenuComponent.dispatchEventImpl(MenuComponent.java:337)
at java.awt.MenuComponent.dispatchEvent(MenuComponent.java:325)
at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:646)
at java.awt.EventQueue.access$000(EventQueue.java:84)
at java.awt.EventQueue$1.run(EventQueue.java:602)
at java.awt.EventQueue$1.run(EventQueue.java:600)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:98)
at java.awt.EventQueue$2.run(EventQueue.java:616)
at java.awt.EventQueue$2.run(EventQueue.java:614)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:613)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)
BUILD SUCCESSFUL (total time: 6 seconds)
答案 0 :(得分：3)
根问题似乎与您实例化InventoryTrackingSystem的两个实例有关。设置一个实例中的cb字段，而另一个实例中的相同字段仍为空。我不确定你为什么要设置这个类的两个实例，但我怀疑你是否会检查你是否会发现你的问题。
祝你好运。答案 1 :(得分：1)
如果我怀疑你的NPE是从这两行之一中抛出的：
if(thisAction.equals("main")){
cb.setSelectedItem(MAINPANEL);
然后thisAction为空，或cb为空。检查出来然后再回来更新。祝好运！
答案 2 :(得分：0)
@colby是对的..您创建的第二个实例setGUI没有创建cb对象。如果您的目的只是调用addComponentToPane（）来构造不同的面板并添加到卡中，那么您只需按如下方式调用，而无需创建对象
addComponentToPane（frame.getContentPane（））;