你可以使用Java Swing来实现按下空格键切换单选按钮的功能。下面是一个简单的示例代码：

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;

public class RadioButtonExample extends JFrame implements ActionListener, KeyListener {
    private JRadioButton radioButton1;
    private JRadioButton radioButton2;

    public RadioButtonExample() {
        // 设置窗口标题
        setTitle("RadioButton Example");
        // 设置窗口大小
        setSize(300, 200);
        // 设置窗口关闭按钮的默认操作
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // 创建单选按钮
        radioButton1 = new JRadioButton("Option 1");
        radioButton2 = new JRadioButton("Option 2");

        // 创建按钮组并将单选按钮添加到按钮组中
        ButtonGroup buttonGroup = new ButtonGroup();
        buttonGroup.add(radioButton1);
        buttonGroup.add(radioButton2);

        // 创建面板并将单选按钮添加到面板中
        JPanel panel = new JPanel();
        panel.add(radioButton1);
        panel.add(radioButton2);

        // 将面板添加到窗口的内容面板中
        getContentPane().add(panel);

        // 添加动作监听器
        radioButton1.addActionListener(this);
        radioButton2.addActionListener(this);

        // 添加键盘监听器
        addKeyListener(this);
        setFocusable(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                new RadioButtonExample().setVisible(true);
            }
        });
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        // 处理单选按钮的点击事件
        if (e.getSource() == radioButton1) {
            System.out.println("Option 1 selected");
        } else if (e.getSource() == radioButton2) {
            System.out.println("Option 2 selected");
        }
    }

    @Override
    public void keyTyped(KeyEvent e) {
        // 按键事件：按下空格键切换单选按钮
        if (e.getKeyChar() == ' ') {
            if (radioButton1.isSelected()) {
                radioButton2.setSelected(true);
            } else {
                radioButton1.setSelected(true);
            }
        }
    }

    @Override
    public void keyPressed(KeyEvent e) {

    }

    @Override
    public void keyReleased(KeyEvent e) {

    }
}

这个示例代码创建了一个带有两个单选按钮的窗口。按下空格键时，会切换选中的单选按钮。当单选按钮被点击时，会打印出选中的选项。