Invisible Radiobutton

[jinnejeevansai]

Hii,i was writing a code for making radiobutton but the yes button is invisible and becomes visible if i move cursor on it,Can someone explain mistake in my code.

import javax.swing.*;
class r{
        public static void main(String args[])
        {
                JFrame f = new JFrame();
                f.setSize(200,200);
                JRadioButton b1 = new JRadioButton("Yes");
                JRadioButton b2 = new JRadioButton("No");
                b1.setBounds(30,50,50,20);
                b2.setBounds(30,80,50,20);
                f.add(b1);
                f.add(b2);
                ButtonGroup bg = new ButtonGroup();
                bg.add(b1);
                bg.add(b2);
                f.setVisible(true);
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        }
}

[jinnejeevansai]

I got the correct output by writing f.setLayout(NUL L); before the line f.setVisible(tr ue); but i was not getting correct output if i wrote below setVisible line.Can someone tell why