How to add a radio button group in a core java program such that only one radio button is selected at one time?

Kindly try using ButtonGroup component and add two JRadioButton components named male and female to the ButtonGroup object and then display it in a JFrame using setVisible(true); method.

The Below code should be useful :-

import java.awt.BorderLayout;
import java.awt.FlowLayout;
import javax.swing.ButtonGroup;
import javax.swing.JFrame;
import javax.swing.JRadioButton;

public class Rb extends JFrame {
    Rb() {
        JRadioButton male = new JRadioButton("male");
        JRadioButton female = new JRadioButton("Female");
        ButtonGroup bG = new ButtonGroup();
        bG.add(male);
        bG.add(female);
        this.setSize(100, 200);
        this.setLayout(new FlowLayout());
        this.add(male);
        this.add(female);
        male.setSelected(true);
        this.setVisible(true);
    }

    public static void main(String args[]) {
        Rb j = new Rb();
    }
}

Here's a radio button grouping:

JRadioButton button1 = ...;
button1.setSelected(true);
JRadioButton button2 = ...;
ButtonGroup group = new ButtonGroup();
group.add(button1);
group.add(button2);

    JPanel radioButtonPanel = new JPanel();
    append = new JRadioButton("append");
    build = new JRadioButton("x.x.1");
    build.setSelected(true); //sets this button as selected on startup
    small = new JRadioButton("x.1.x");
    huge = new JRadioButton("1.x.x");

    // Create the button group to keep only one selected.
    ButtonGroup btnGroup = new ButtonGroup();
    btnGroup.add(append);
    btnGroup.add(build);
    btnGroup.add(small);
    btnGroup.add(huge);

Then you add your Buttons to your JPanel or something similar.