Código de Java - Mostrar la hora a partir de una hora dada

import java.util.Observable;

import java.util.Observer;

import java.util.logging.Level;

import java.util.logging.Logger;

public class FrmContador extends javax.swing.JFrame implements Observer{

    public FrmContador() {

        initComponents();

    }

    @SuppressWarnings("unchecked")

    private void initComponents() {

        lblCronometro = new javax.swing.JLabel();

        btnIniciar = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        setTitle("Reloj digital");

        lblCronometro.setFont(new java.awt.Font("Tahoma", 1, 48)); // NOI18N

        btnIniciar.setFont(new java.awt.Font("Tahoma", 0, 18)); // NOI18N

        btnIniciar.setText("Iniciar");

        btnIniciar.addActionListener(new java.awt.event.ActionListener() {

            public void actionPerformed(java.awt.event.ActionEvent evt) {

                btnIniciarActionPerformed(evt);

            }

        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());

        getContentPane().setLayout(layout);

        layout.setHorizontalGroup(

            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)

            .addGroup(layout.createSequentialGroup()

                .addGap(22, 22, 22)

                .addComponent(lblCronometro, javax.swing.GroupLayout.PREFERRED_SIZE, 246, javax.swing.GroupLayout.PREFERRED_SIZE)

                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)

                .addComponent(btnIniciar, javax.swing.GroupLayout.DEFAULT_SIZE, 97, Short.MAX_VALUE)

                .addContainerGap())

        );

        layout.setVerticalGroup(

            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)

            .addGroup(layout.createSequentialGroup()

                .addContainerGap()

                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)

                    .addComponent(lblCronometro, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)

                    .addComponent(btnIniciar, javax.swing.GroupLayout.DEFAULT_SIZE, 129, Short.MAX_VALUE))

                .addContainerGap())

        );

        pack();

    }

    private void btnIniciarActionPerformed(java.awt.event.ActionEvent evt) {//GEN-FIRST:event_btnIniciarActionPerformed

        this.btnIniciar.setEnabled(false);

        RelojDigital r = new RelojDigital(23, 59, 50);

        r.addObserver(this);

        Thread t = new Thread(r);

        t.start();

    }

    /**

     * @param args the command line arguments

     */

    public static void main(String args[]) {

        /* Set the Nimbus look and feel */

        /* 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(FrmContador.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);

        } catch (InstantiationException ex) {

            java.util.logging.Logger.getLogger(FrmContador.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);

        } catch (IllegalAccessException ex) {

            java.util.logging.Logger.getLogger(FrmContador.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);

        } catch (javax.swing.UnsupportedLookAndFeelException ex) {

            java.util.logging.Logger.getLogger(FrmContador.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);

        }

        /* Create and display the form */

        java.awt.EventQueue.invokeLater(new Runnable() {

            public void run() {

                new FrmContador().setVisible(true);

            }

        });

    }

    private javax.swing.JButton btnIniciar;

    private javax.swing.JLabel lblCronometro;

    @Override

    public void update(Observable o, Object arg) {

        lblCronometro.setText((String) arg);

    }

}

class RelojDigital extends Observable implements Runnable {

    private int horas, minutos, segundos;

    public RelojDigital(int horas, int minutos, int segundos) {

        this.horas = horas;

        this.minutos = minutos;

        this.segundos = segundos;

    }

    @Override

    public void run() {

        String tiempo;

        try {

            while (true) {

                tiempo = "";

                if (horas < 10) {

                    tiempo += "0" + horas;

                } else {

                    tiempo += horas;

                }

                tiempo += ":";

                if (minutos < 10) {

                    tiempo += "0" + minutos;

                } else {

                    tiempo += minutos;

                }

                tiempo += ":";

                if (segundos < 10) {

                    tiempo += "0" + segundos;

                } else {

                    tiempo += segundos;

                }

                this.setChanged();

                this.notifyObservers(tiempo);

                this.clearChanged();

                Thread.sleep(1000);

                segundos++;

                if (segundos == 60) {

                    minutos++;

                    segundos = 0;

                    if (minutos == 60) {

                        minutos = 0;

                        horas++;

                        if (horas == 24) {

                            horas = 0;

                        }

                    }

                }

            }

        } catch (InterruptedException ex) {

            Logger.getLogger(RelojDigital.class.getName()).log(Level.SEVERE, null, ex);

        }

    }

}