Java Swing KeyListener secret code example

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
/* Program to show how to detect one letter at a time in a JTextField */

public class SecretCode2 extends JFrame implements KeyListener {

    Font displayFont = new Font("Arial", Font.PLAIN, 18);
    JTextField display;
    JLabel d2;
    JPanel panel;

    String s = "GFT";
    char[] c = {'*','*','*'};
    char[] d = {'*','*','*'};
    String ds = "~!@#$%^&*()?.,";

    public static void main(String[] args) {
        new SecretCode2();
    }

    SecretCode2() {
        this.setSize(200,130);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setLocationRelativeTo(null);
        this.setTitle("Enter Secret Code");
        this.setResizable(false);

        panel = new JPanel();
        panel.setLayout(new FlowLayout(FlowLayout.CENTER,10,10));
        panel.setBackground(new Color(70,70,90));
        display = new JTextField("***",9);
        display.setHorizontalAlignment(JTextField.CENTER);
        display.setFont(displayFont);
        display.addKeyListener(this);
        panel.add(display);
        d2 = new JLabel("   ***   ");
        d2.setPreferredSize(new Dimension(126,26));
        d2.setBackground(Color.WHITE);
        d2.setOpaque(true);
        d2.setHorizontalAlignment(JTextField.CENTER);
        d2.setFont(displayFont);

        panel.add(d2);
        this.add(panel);
        this.setVisible(true);
    }


    @Override
    public void keyTyped( KeyEvent e ) {
        char key = e.getKeyChar();
        // if ((int)key == 8)   return;
        if ((key >='A' && key <='Z') || (key >='a' && key <= 'z')) {}
        else {
            e.consume();
            return;
        }

        c[0]=c[1];
        c[1]=c[2];
        c[2]=key;

        d[0]=d[1];
        d[1]=d[2];
        d[2]=ds.charAt((int)(Math.random() * ds.length()));

        String t = new String(c).toUpperCase();
        display.setText(t);
        d2.setText(new String(d));
        //d2.setText(" "+(int)key);
        e.consume();    //after it updates the display, the keypressed also is displayed. Must consume it!!!

        if (t.equals(s)) {
            panel.setBackground(Color.RED);
            display.setText("YOU WIN");
            d2.setText("$$$");
        }
    }
    @Override
    public void keyReleased( KeyEvent e ) {}
    @Override
    public void keyPressed( KeyEvent e ) {}
}