Empty JTextField ?

Hello,
I have a weird problem that I haven't solve. In my program, I would
like print in the terminal the value of the JTextField fieldNom when I
click on the button "Lancer la modification". But it always print an
empty string...
Here's the code :

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.*;

public class Fenetre extends JFrame implements ActionListener {
    private Container container;

    private JLabel labelNom;
    private JLabel labelVieuxMdp;
    private JLabel labelNouveauMdp;
    private JLabel labelConfirmerMdp;

    private JTextField fieldNom;
    private JPasswordField fieldVieuxMdp;
    private JPasswordField fieldNouveauMdp;
    private JPasswordField fieldConfirmerMdp;

    private JButton buttonLancer;

    public Fenetre() {
        super();
        setTitle("Changer mot de passe");
        setSize(350,150);
        setLocation(200,200);
        setContentPane(construireFenetre());
        setVisible(true);

    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                Fenetre f = new Fenetre();
                f.construireFenetre();
            }
        });
    }

    public void actionPerformed(ActionEvent e) {
        System.out.println("nom : "+getNom());
    }

    private Container construireFenetre() {
        container = new JPanel();
        container.setLayout(new GridBagLayout());
        GridBagConstraints contraintes = new GridBagConstraints();

        labelNom = new JLabel();
        labelNom.setText("Nom");
        contraintes.fill = GridBagConstraints.HORIZONTAL;
        contraintes.weightx = 0.5;
        contraintes.gridx = 0;
        contraintes.gridy = 0;
        container.add(labelNom, contraintes);

        fieldNom = new JTextField(25);
        contraintes.fill = GridBagConstraints.HORIZONTAL;
        contraintes.weightx = 0.5;
        contraintes.gridx = 1;
        contraintes.gridy = 0;
        container.add(fieldNom, contraintes);

        labelVieuxMdp = new JLabel();
        labelVieuxMdp.setText("Mot de passe");
        contraintes.fill = GridBagConstraints.HORIZONTAL;
        contraintes.weightx = 0.5;
        contraintes.gridx = 0;
        contraintes.gridy = 1;
        container.add(labelVieuxMdp, contraintes);

        fieldVieuxMdp = new JPasswordField();
        contraintes.fill = GridBagConstraints.HORIZONTAL;
        contraintes.weightx = 0.5;
        contraintes.gridx = 1;
        contraintes.gridy = 1;
        container.add(fieldVieuxMdp, contraintes);

        labelNouveauMdp = new JLabel();
        labelNouveauMdp.setText("Nouveau mot de passe");
        contraintes.fill = GridBagConstraints.HORIZONTAL;
        contraintes.weightx = 0.5;
        contraintes.gridx = 0;
        contraintes.gridy = 2;
        container.add(labelNouveauMdp, contraintes);

        fieldNouveauMdp = new JPasswordField();
        contraintes.fill = GridBagConstraints.HORIZONTAL;
        contraintes.weightx = 0.5;
        contraintes.gridx = 1;
        contraintes.gridy = 2;
        container.add(fieldNouveauMdp, contraintes);

        labelConfirmerMdp = new JLabel();
        labelConfirmerMdp.setText("Retapez le mot de passe");
        contraintes.fill = GridBagConstraints.HORIZONTAL;
        contraintes.weightx = 0.5;
        contraintes.gridx = 0;
        contraintes.gridy = 3;
        container.add(labelConfirmerMdp, contraintes);

        fieldConfirmerMdp = new JPasswordField();
        contraintes.fill = GridBagConstraints.HORIZONTAL;
        contraintes.weightx = 0.5;
        contraintes.gridx = 1;
        contraintes.gridy = 3;
        container.add(fieldConfirmerMdp, contraintes);

        buttonLancer = new JButton();
        buttonLancer.setText("Lancer la modification");
        contraintes.fill = GridBagConstraints.HORIZONTAL;
        contraintes.weightx = 0.0;
        contraintes.gridwidth = 2;
        contraintes.gridx = 0;
        contraintes.gridy = 4;
        buttonLancer.addActionListener(this);
        container.add(buttonLancer, contraintes);

        return container;
    }

    public String getNom() {
        return fieldNom.getText();
    }

    public String getVieuxMdp() {
        return new String(fieldVieuxMdp.getPassword());
    }

    public String getNouveauMdp() {
        return new String(fieldNouveauMdp.getPassword());
    }

    public String getConfirmerMdp() {
        return new String(fieldConfirmerMdp.getPassword());
    }

}

Thanks,
Vincent