RadioButtonFrame.java

package radioButton;

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

/**
 * A frame with a sample text label and radio buttons for selecting font sizes.
 */
public class RadioButtonFrame extends JFrame {
    public static final int DEFAULT_SIZE = 36;
    private JPanel buttonPanel;
    private ButtonGroup group;
    private JLabel label;

    public RadioButtonFrame() {
        // add the sample text label

        label = new JLabel("The quick brown fox jumps over the lazy dog");
        label.setFont(new Font("Serif", Font.PLAIN, DEFAULT_SIZE));
        add(label, BorderLayout.CENTER);

        // add the ration buttons

        buttonPanel = new JPanel();
        group = new ButtonGroup();

        addRadioButton("small", 8);
        addRadioButton("Medium", 12);
        addRadioButton("Large", 18);
        addRadioButton("Extra Large", 36);

        add(buttonPanel, BorderLayout.SOUTH);
        pack();
    }

    /**
     * Adds a ratio button that sets the font size of the sample text
     *
     * @param name the string to appear on the button
     * @param size size the font size that this button sets
     */
    private void addRadioButton(String name, int size) {
        boolean selected = size == DEFAULT_SIZE;
        JRadioButton button = new JRadioButton(name, selected);
        group.add(button);
        buttonPanel.add(button);

        // this listener sets the label font size

        ActionListener listener = e -> label.setFont(new Font("Serif", Font.PLAIN, size));

        button.addActionListener(listener);
    }
}