Interfaz cálculo DNI sencilla "tipo examen"

En un ejemplo real controlad las pulsaciones para permitir sólo dígitos:

/*
* To change this template, choose Tools | Templates
* and open the template in the editor.
*/
package dni;

import javax.swing.JOptionPane;

/**
*
* @author juan
*/
public class AplicacionDNI extends javax.swing.JFrame {

    public static final String NIF_STRING_ASOCIATION = "TRWAGMYFPDXBNJZSQVHLCKE";

  /**
   * Devuelve un NIF completo a partir de un DNI. Es decir, añade la letra del NIF
   * @param dni dni al que se quiere añadir la letra del NIF
   * @return NIF completo.
   */
  public static String letraDNI(int dni) {
    return String.valueOf(dni) + NIF_STRING_ASOCIATION.charAt(dni % 23);
  }
    /**
     * Creates new form AplicacionDNI
     */
    public AplicacionDNI() {
        initComponents();
    }


    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">
    private void initComponents() {

        txtDNI = new javax.swing.JTextField();
        btnCalcular = new javax.swing.JButton();
        lblNIF = new javax.swing.JLabel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        btnCalcular.setText("Calcular");
        btnCalcular.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                btnCalcularActionPerformed(evt);
            }
        });

        lblNIF.setBackground(new java.awt.Color(51, 51, 51));

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
                    .addComponent(lblNIF, javax.swing.GroupLayout.PREFERRED_SIZE, 227, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                        .addGroup(layout.createSequentialGroup()
                            .addGap(82, 82, 82)
                            .addComponent(txtDNI, javax.swing.GroupLayout.PREFERRED_SIZE, 227, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addGroup(layout.createSequentialGroup()
                            .addGap(148, 148, 148)
                            .addComponent(btnCalcular))))
                .addContainerGap(91, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGap(30, 30, 30)
                .addComponent(txtDNI, javax.swing.GroupLayout.PREFERRED_SIZE, 37, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(46, 46, 46)
                .addComponent(btnCalcular)
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 45, Short.MAX_VALUE)
                .addComponent(lblNIF, javax.swing.GroupLayout.PREFERRED_SIZE, 44, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(75, 75, 75))
        );

        pack();
    }// </editor-fold>

    private void btnCalcularActionPerformed(java.awt.event.ActionEvent evt) {
        try {
                    lblNIF.setText(letraDNI(Integer.parseInt(txtDNI.getText())));        // TODO add your handling code here:

        } catch (Exception e) {
            JOptionPane.showMessageDialog(null, "Se ha producido un error de conversión: "+e);
        }
    }

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
        /*
         * Set the Nimbus look and feel
         */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /*
         * If Nimbus (introduced in Java SE 6) is not available, stay with the
         * default look and feel. For details see
         * http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(AplicacionDNI.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(AplicacionDNI.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(AplicacionDNI.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(AplicacionDNI.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /*
         * Create and display the form
         */
        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {
                new AplicacionDNI().setVisible(true);
            }
        });
    }
    // Variables declaration - do not modify
    private javax.swing.JButton btnCalcular;
    private javax.swing.JLabel lblNIF;
    private javax.swing.JTextField txtDNI;
    // End of variables declaration
}



Fuente:
Algoritmo cálculo DNI: http://es.wikibooks.org/wiki/Algoritmo_para_obtener_la_letra_del_NIF