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？？

参考代码：

import javax.swing.*;
import java.awt.*;

public class Test {
    public static void main(String[] args) {
        ButtonGroup btnGroup = new ButtonGroup();
        JMenuBar menuBar = new JMenuBar();
        JMenu menu = new JMenu("Items");
        
        menuBar.add(menu);
        JRadioButtonMenuItem[] items = {
            new JRadioButtonMenuItem("Item1"),
            new JRadioButtonMenuItem("Item2"),
        };
        for (int i = 0; i < items.length; ++i) {
            menu.add(items[i]);
            btnGroup.add(items[i]);
        }
        items[0].setSelected(true); // 设置第一个单选项item1为选中状态
        
        JFrame frame = new JFrame("Test");
        frame.setJMenuBar(menuBar);
        frame.setLayout(new FlowLayout());
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setBounds(200, 200, 300, 300);
        frame.setVisible(true);
    }
}