Lower case to Upper case conversion in java

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

//////////////////////////////////////////////////////////////// ToUpperCase
public class ToUpperCase extends JApplet {

    //=================================================== instance variables
    private JTextField _inField  = new JTextField(20);
    private JTextField _outField = new JTextField(20);

    //================================================================= main
    public static void main(String[] args) {
        JFrame window = new JFrame();
        window.setTitle("ToUpperCase Example");
        window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        
        //... JApplet works fine as content pane in a window!
        window.setContentPane(new ToUpperCase());

        window.pack();                       // Layout components.
        window.setLocationRelativeTo(null);  // Center window.
        window.setVisible(true);
    }

    //================================================== applet constructor
    public ToUpperCase() {
        //... Create or set attributes of components.
        _outField.setEditable(false);    // Don't let user change output.
        JButton toUpperButton = new JButton("To Uppercase");

        //... Add listener to button.
        toUpperButton.addActionListener(new UpperCaseAction());

        //... Add components directly to applet.  Don't need content pane.
        setLayout(new FlowLayout());
        add(_inField);
        add(toUpperButton);
        add(_outField);
    }

    /////////////////////////////////// inner listener class UpperCaseAction
    class UpperCaseAction implements ActionListener {
        public void actionPerformed(ActionEvent e) {
            //... Convert text in one textfield to uppercase in another.
            String data = _inField.getText();  // Get the text
            String out  = data.toUpperCase();  // Create uppercase version.
            _outField.setText(out);            // Set output field
        }
    }
}