That really isn't an SSCCE. Here is an SSCCE that shows that setSelected() does indeed work:
import javax.swing.JFrame;
import javax.swing.JRadioButton;
import java.awt.FlowLayout;
public class ButtonTest {
public ButtonTest() {
JFrame frame = new JFrame("Button Test");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setSize(200,200);
JRadioButton button = new JRadioButton("JRadioButton");
button.setSelected(true);
frame.setLayout(new FlowLayout());
frame.add(button);
frame.setVisible(true);
}
public static void main(String[] args) {
new ButtonTest();
}
}