Swing JComboBox 动态增加的时候，下拉箭头没显示问题

原来，在动态增加JComboBox 的时候，如果没增加validate（）方法 仅仅是使用了 repaint（）方法的话，JComboBox的下拉箭头可能会不显示。如：

button_2.addActionListener(new ActionListener() {public void actionPerformed(ActionEvent e) {comdef = new ComponentDefined(x,20 + count_add * 20);
//				 JComboBox j1 =  new JComboBox(new String[]{"","OR","AND"});
//				 j1.setBounds(x, 20 + count_add* 30, 55, 20);
//				 comdef.setComBox(j1);
//				comdef.getComBox().updateUI();panel.add(comdef.getComBox());panel.add(comdef.getTextField());
//				panel.validate();panel.repaint();count_add =  count_add +1;System.out.println(panel.getComponentCount());}});