java - 刷新 JComboBox 抛出异常

Question

我有一个组合框，它使用带有监听器的模型来捕获所选项目：

deleteCombo = new JComboBox<Course>(course.toArray(new Course[course.size()]));
model = new DefaultComboBoxModel<Course>( course.toArray(new Course[course.size()]) );
deleteCombo.setModel( model );
deleteCombo.setRenderer(renderer);
deleteCombo.setSelectedIndex(0);
deleteCombo.addActionListener( new ActionListener() {

    public void actionPerformed(ActionEvent e)
    {
        JComboBox deleteCombo = (JComboBox)e.getSource();
        Course course = (Course)deleteCombo.getSelectedItem();
        deleteNumTxt.setText( course.getCourseNum() ); //This is line 391
        deleteTitleTxt.setText( course.getCourseTitle() );
        deleteDurationTxt.setText( String.valueOf(course.getDuration() ) );
        deleteFeeTxt.setText("$" + String.valueOf(course.getCourseFee() ) );
        deleteLevelTxt.setText( course.getCourseLevel() );
    }
});

我正在使用 itemSelected 事件来填充 GUI 中的文本字段，以显示所选课程的详细信息。选择课程并单击“删除课程”按钮后，将转到此方法：

public static void deleteCourseRecord() {
courseNumIn = deleteNumTxt.getText();
int index = course.indexOf(new Course(courseNumIn));
course.remove(index);

//RE-POPULATE EDIT AND DELETE COMBOBOXES
model.removeAllElements();
for(Course item : course) {
    model.addElement(item);
    }
}

记录被删除但抛出此异常：

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at MyCourse$2.actionPerformed(MyCourse.java:391)
at javax.swing.JComboBox.fireActionEvent(JComboBox.java:1260)
at javax.swing.JComboBox.contentsChanged(JComboBox.java:1331)
at javax.swing.JComboBox.intervalRemoved(JComboBox.java:1351)
at javax.swing.AbstractListModel.fireIntervalRemoved(AbstractListModel.java:179)
at javax.swing.DefaultComboBoxModel.removeAllElements(DefaultComboBoxModel.java:175)
at MyCourse$Course.deleteCourseRecord(MyCourse.java:1138)
at MyCourse.actionPerformed(MyCourse.java:883)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
tjavax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)

我认为构建 actionPerformed 部分的方式存在问题，但我不知道它是什么。如果我不使用模型（或任何其他方法）刷新组合框，它不会抛出异常。如果我注释掉第 391 行（在代码中标记），它将在第 392 行抛出异常，依此类推。如果填充文本字段的所有代码行都被注释掉，它不会抛出异常，但当然，函数会丢失。关于我做错了什么的任何想法？干杯

Answer 1

问题如下：

每次更改设置为组合框的模型时，都会调用动作侦听器。

所以当你这样做时：

//RE-POPULATE EDIT AND DELETE COMBOBOXES
model.removeAllElements();

actionlistener 被执行并且

Course course = (Course)deleteCombo.getSelectedItem();

为空，因为您刚刚从组合框中删除了所有元素。

如果你试试这个简单的例子，你会看到：

public static void main(String[] args) 
    String [] s = {"1", "2"};
    DefaultComboBoxModel<String> model = new DefaultComboBoxModel<String>(s);
    JComboBox<String> j = new JComboBox<String>(s);
    j.setModel(model);

    j.addActionListener(new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent arg0) {
            System.out.println(((JComboBox) arg0.getSource()).getSelectedItem());               
        }
    });

    JFrame jf = new JFrame();
    jf.add(j);
    jf.setVisible(true);
    jf.setSize(100, 100);

    model.removeAllElements();
}

它将输出null。