1.JRadioButtonMenuItem  mi3=new  JRadioButtonMenuItem("Open");
   JRadioButtonMenuItem  mi4=new  JRadioButtonMenuItem("Close");  这是两个单选菜单项，如何让第一个在一开始就处于选中状态？？2
2JPanel  panel3=new  JPanel();
        ButtonGroup bg1=new ButtonGroup();
        panel3.add(new  JLabel("JRadioButton:"));
        String[]JRadioButton={"JRadioButton1","JRadioButton2","JRadioButton3"};  如何一开始让JRadioButton1处于选中状态？？