I have the following code snippets. Both has a method call to
addWindowListener

It is my understanding that BookOrderFrame is using a callback method
to implement the interface for WindowListener. Am I correct?

LoanCalculatorFrame otoh, does it substantially different and does not
even state "implements WindowListener".

So can anyone please explain what LoanCalculatorFrame does and whether
and how it is better/worse than what BookOrderFrame is doing?

Thanks.

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

public class LoanCalculatorFrame extends JFrame
{
    public LoanCalculatorFrame()
    {
        setTitle("Loan Calculator");
        Toolkit tk = Toolkit.getDefaultToolkit();
        Dimension d = tk.getScreenSize();
        System.out.println("Screen:" + d.width + " by " + d.height + "
pixels");
        int height = 200;
        int width = 267;
        setBounds((d.width-width)/2, (d.height-height)/2, width, height);
        setResizable(false);
        addWindowListener(new WindowAdapter()
                            {
                                public void windowClosing(WindowEvent e)
                                {
                                    System.exit(0);
                                }
                            }
                        );
        Container contentPane = getContentPane();
        JPanel panel = new JPanel();
        contentPane.add(panel);

    }
}

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

public class BookOrderFrame extends JFrame implements WindowListener
{
    public BookOrderFrame()
    {
        setTitle("Book Order");
        setBounds(267, 200, 267, 200);
        addWindowListener(this);
    }

    public void windowDeactivated(WindowEvent e)
    {
    }

    public void windowActivated(WindowEvent e)
    {
    }

    public void windowDeiconified(WindowEvent e)
    {
    }

    public void windowIconified(WindowEvent e)
    {
    }

    public void windowClosed(WindowEvent e)
    {
    }

    public void windowClosing(WindowEvent e)
    {
        System.exit(0);
    }

    public void windowOpened(WindowEvent e)
    {
    }
}