创建自定义JComboBox

Question

这是我第一次尝试创建自定义swing组件。我正在尝试创建一个JComboBox，它将用于JTable中的众多单元。显示我的JTable时，应该包含组合框的单元格看起来是正确的。 但是，当我单击任何框时，列表不会显示。什么都没发生。这是我的代码：

public class TimeLineComboBox extends JComboBox<String> {

/**
 * 
 */
private static final long serialVersionUID = 1L;

private String item="";
private TimelineRecord timeLine=null;

public TimeLineComboBox(String[] list_,TimelineRecord timeLine_,String item_,Boolean editable_) {
    super(list_);
    this.addActionListener(listener());
    timeLine=timeLine_;
    item=item_;
}

private ActionListener listener() {
    ActionListener actionListener=new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent e) {
            @SuppressWarnings("unchecked")
            String value=(String) ((JComboBox<String>) e.getSource()).getSelectedItem();
            switch(item) {
                case "meat":
                    timeLine.meat=value;
                    break;
                case "monitor":
                    timeLine.monitor=Configuration.getInstance().getMonitorByName(value);
                    break;
                case "pit":
                    timeLine.pit=Configuration.getInstance().getMonitorByName(value);
                    break;
                case "event":
                    timeLine.eventText=value;
                    break;
                default:
                    JOptionPane.showMessageDialog(GUI.getInstance().getFrame(),"Internal Error: Unknown selection list: "+item,"Internal Error",JOptionPane.ERROR_MESSAGE);
            }
        }

    };
    return(actionListener);
}

}