Question

我正在创建一个组合框，它一直给我一个我不理解的项目监听器的错误。如果你也可以解释一下你的答案会很好。提前致谢：

这是组合框的项目监听器：

combo1.addItemListener(new ItemListener(){
            @Override
            public void itemStateChanged(ItemEvent e){
               if(e.getStateChange() == ItemEvent.SELECTED){
                  JComboBox localCombo = (JComboBox)e.getSource();
                  ic1[0] = localCombo.getSelectedItem().toString();     
               }  
            }
        });
        combo2.addItemListener(new ItemListener(){
            @Override
            public void itemStateChanged(ItemEvent e){

                if(e.getStateChange() == ItemEvent.SELECTED){
                    JComboBox localCombo = (JComboBox)e.getSource();
                    ic1[1] = localCombo.getSelectedItem().toString();
                }
            }
        });

错误表明它位于.getSelectedItem（）...行上。它只会在我运行程序并在框中选择一个单词时给出错误。谢谢！

这是运行中的错误（第61行是.getSelectedItem（）...行）：

    run:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
    at thisprogramisforfun.guiClasses.guiClassConversions$1.itemStateChanged(guiClassConversions.java:61)
    at javax.swing.JComboBox.fireItemStateChanged(JComboBox.java:1225)
    at javax.swing.JComboBox.selectedItemChanged(JComboBox.java:1282)
    at javax.swing.JComboBox.contentsChanged(JComboBox.java:1329)
    at javax.swing.AbstractListModel.fireContentsChanged(AbstractListModel.java:118)
    at javax.swing.DefaultComboBoxModel.setSelectedItem(DefaultComboBoxModel.java:93)
    at javax.swing.JComboBox.setSelectedItem(JComboBox.java:578)
    at javax.swing.JComboBox.setSelectedIndex(JComboBox.java:624)
    at javax.swing.plaf.basic.BasicComboPopup$Handler.mouseReleased(BasicComboPopup.java:835)
    at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:290)
    at java.awt.Component.processMouseEvent(Component.java:6505)
    at javax.swing.JComponent.processMouseEvent(JComponent.java:3320)
    at javax.swing.plaf.basic.BasicComboPopup$1.processMouseEvent(BasicComboPopup.java:499)
    at java.awt.Component.processEvent(Component.java:6270)
    at java.awt.Container.processEvent(Container.java:2229)
    at java.awt.Component.dispatchEventImpl(Component.java:4861)
    at java.awt.Container.dispatchEventImpl(Container.java:2287)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
    at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
    at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
    at java.awt.Container.dispatchEventImpl(Container.java:2273)
    at java.awt.Window.dispatchEventImpl(Window.java:2719)
    at java.awt.Component.dispatchEvent(Component.java:4687)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:735)
    at java.awt.EventQueue.access$200(EventQueue.java:103)
    at java.awt.EventQueue$3.run(EventQueue.java:694)
    at java.awt.EventQueue$3.run(EventQueue.java:692)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
    at java.awt.EventQueue$4.run(EventQueue.java:708)
    at java.awt.EventQueue$4.run(EventQueue.java:706)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:705)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:242)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:161)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:150)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:146)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:138)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:91)

Answer 1

你的问题在这里：

combo1 = (JComboBox)e.getSource();
ic1[0] = combo2.getSelectedItem().toString();

您正在获取combo1但在combo2上调用方法

更好的是：

combo1 = (JComboBox)e.getSource();
ic1[0] = combo1.getSelectedItem().toString();

但更好的是，使用局部变量进行此类工作，而不是字段。

JComboBox localCombo = (JComboBox)e.getSource();
ic1[0] = localCombo.getSelectedItem().toString();

Answer 2

错误表明它位于.getSelectedItem（）...行
上

所以你在这条线上只有两个变量：

ic1阵列
localCombo变量

您知道如何使用System.out.println（...）来显示这些变量的值吗？

我在ici数组中的猜测没有初始化，因为你从事件源获得了localCombo变量。

JComboBox项目侦听器选定项目

2 个答案: