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jComboBox.removeAllItems在添加項目（在從配置文件填充JComboBox之前調用一次）之後纔會投訴。遍歷這段代碼，我看到jComboBox.removeAllItems調用JComboBox.removeAllItems，它調用了調用fireIntervalRemoved的DefaultComboBoxModel.removeAllElements。 jComboBox中有七個項目，並且在拋出NullPointerException之前，fireIntervalRemoved中的循環大約執行了四次。有任何想法嗎？爲什麼JComboBox.removeAllItems（）拋出NullPointerException？
public class MyClass extends javax.swing.JFrame {
...
private void updateComboBox() {
try {
jComboBox.removeAllItems();
...
public class JComboBox ...
...
public void removeAllItems() {
checkMutableComboBoxModel();
MutableComboBoxModel model = (MutableComboBoxModel)dataModel;
int size = model.getSize();
if (model instanceof DefaultComboBoxModel) {
((DefaultComboBoxModel)model).removeAllElements();
...
public class DefaultComboBoxModel ...
...
public void removeAllElements() {
if (objects.size() > 0) {
int firstIndex = 0;
int lastIndex = objects.size() - 1;
objects.removeAllElements();
selectedObject = null;
fireIntervalRemoved(this, firstIndex, lastIndex);
...
public abstract class AbstractListModel ...
...
protected void fireIntervalRemoved(Object source, int index0, int index1)
{
Object[] listeners = listenerList.getListenerList();
ListDataEvent e = null;
for (int i = listeners.length - 2; i >= 0; i -= 2) {
if (listeners[i] == ListDataListener.class) {
if (e == null) {
e = new ListDataEvent(source,
ListDataEvent.INTERVAL_REMOVED, index0, index1);
}
((ListDataListener)listeners[i+1]).intervalRemoved(e);
...
顯示java.lang.NullPointerException
at abc.MyClass.jComboBoxActionPerformed(MyClass.java:211)
at abc.MyClass.access$000(MyClass.java:16)
at abc.MyClass$1.actionPerformed(MyClass.java:110)
at javax.swing.JComboBox.fireActionEvent(JComboBox.java:1240)
at javax.swing.JComboBox.contentsChanged(JComboBox.java:1311)
at javax.swing.JComboBox.intervalRemoved(JComboBox.java:1331)
at javax.swing.AbstractListModel.fireIntervalRemoved(AbstractListModel.java:161)
at javax.swing.DefaultComboBoxModel.removeAllElements(DefaultComboBoxModel.java:169)
at javax.swing.JComboBox.removeAllItems(JComboBox.java:751)
at abc.MyClass.updateComboBox(MyClass.java:58)
at abc.MyClass.jMenuItemDevicesDeleteActionPerformed(MyClass.java:231)
at abc.MyClass.access$300(MyClass.java:16)
at abc.MyClass$4.actionPerformed(MyClass.java:154)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:1995)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2318)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:387)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:242)
at javax.swing.AbstractButton.doClick(AbstractButton.java:357)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:1223)
at javax.swing.plaf.basic.BasicMenuItemUI$Handler.mouseReleased(BasicMenuItemUI.java:1264)
at java.awt.Component.processMouseEvent(Component.java:6267)
at javax.swing.JComponent.processMouseEvent(JComponent.java:3267)
at java.awt.Component.processEvent(Component.java:6032)
at java.awt.Container.processEvent(Container.java:2041)
at java.awt.Component.dispatchEventImpl(Component.java:4630)
at java.awt.Container.dispatchEventImpl(Container.java:2099)
at java.awt.Component.dispatchEvent(Component.java:4460)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4577)
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4238)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4168)
at java.awt.Container.dispatchEventImpl(Container.java:2085)
at java.awt.Window.dispatchEventImpl(Window.java:2478)
at java.awt.Component.dispatchEvent(Component.java:4460)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:599)
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)