Ajouter une action a un bouton...

ouvrir.addActionListener(new ActionListener()
{
    @Override
    public void actionPerformed(ActionEvent e)
    {
        Desktop.getDesktop().open(laListe.getSelectedValue());
    }	
});

package projet;

import java.awt.Color;
import java.awt.Component;
import java.awt.Desktop;
import java.awt.GridLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.JMenu;
import javax.swing.JMenuBar;
import javax.swing.JMenuItem;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextField;
import javax.swing.ListSelectionModel;
import javax.swing.border.LineBorder;
import javax.swing.event.ListSelectionEvent;
import javax.swing.event.ListSelectionListener;

import java.io.File;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.NoSuchElementException;

public class ArboIHM extends JFrame implements ActionListener {

	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;
	private List<File> liste = new ArrayList<File>();
	private JPanel panel = new JPanel();
	private JPanel boutons = new JPanel();
	private JButton triNom = new JButton("Arborescence triée par nom");
	private JButton triTaille = new JButton("Arborescence triée par taille");
	// private JButton triDate = new JButton("Arborescence triée par date");
	private File test = new File("C:/Users/");
	private ArboTriee a1 = new ArboTrieeParNom(test);;
	private ArboTriee at1 = new ArboTrieeParTaille(test);
	private JList laListe = new JList(liste.toArray());;
	private JLabel nomLabel = new JLabel("Nom : ");
	private JLabel tailleLabel = new JLabel("Taille : ");
	private JLabel dateLabel = new JLabel("Date : ");
	private JPanel nom = new JPanel();
	private JPanel taille = new JPanel();
	private JPanel date = new JPanel();
	private JTextField nomTexte = new JTextField(
			"Veuillez sélectionner un fichier", 20);
	private JTextField tailleTexte = new JTextField(
			"Veuillez sélectionner un fichier", 20);
	private JTextField dateTexte = new JTextField(
			"Veuillez sélectionner un fichier", 20);
	private JTextField rechercheTexte = new JTextField(
			"Veuillez saisir le nom d'un fichier", 20);
	private JScrollPane scrollPane = new JScrollPane(laListe);
	private JButton ouvrir = new JButton("Ouvrir le fichier");
	private GridLayout g1 = new GridLayout(3, 1);
	private JLabel rechercheLabel = new JLabel("Rechercher : ");
	private JPanel recherche = new JPanel();
	private JScrollPane j1 = new JScrollPane(laListe);
	private JPanel infos = new JPanel();
	private File f1;
	private String str = "";
	private JMenuBar jmb = new JMenuBar();

	private JMenu fichier = new JMenu("Fichier");
	private JMenu aide = new JMenu("?");

	private JMenuItem itemNouveau = new JMenuItem("Nouveau");
	private JMenuItem itemSauver = new JMenuItem("Sauvegarder");
	private JMenuItem itemCharger = new JMenuItem("Charger");
	private JMenuItem itemQuitter = new JMenuItem("Quitter");
	private JMenuItem itemPropos = new JMenuItem("A Propos");
	private JMenuItem itemAide = new JMenuItem("Aide");

	public ArboIHM() {
		init();
	}

	public void init() {
		setVisible(true);
		this.setJMenuBar(this.creationBarreDeMenu());
		setTitle("Projet de JAVA");
		pack();
		setSize(700, 300);
		this.setLocationRelativeTo(null);
		setResizable(false);
		setDefaultCloseOperation(EXIT_ON_CLOSE);
		nomTexte.setEditable(false);
		dateTexte.setEditable(false);
		tailleTexte.setEditable(false);

		add(panel);
		nom.add(nomLabel);
		nom.add(nomTexte);
		date.add(dateLabel);
		date.add(dateTexte);
		taille.add(tailleLabel);
		taille.add(tailleTexte);
		recherche.add(rechercheLabel);
		recherche.add(rechercheTexte);

		boutons.add(triNom);
		boutons.add(triTaille);
		boutons.add(ouvrir);

		infos.add(nom);
		infos.add(date);
		infos.add(taille);
		infos.add(recherche);

		panel.setLayout(g1);
		panel.add(boutons);
		panel.add(infos);
		j1.setViewportView(laListe);
		panel.add(j1);

		triNom.addActionListener(new TriParNom());
		triTaille.addActionListener(new TriParTaille());

		rechercheTexte.addKeyListener(new RechercheFichier());
		rechercheTexte.addMouseListener(new ClicGauche());

		laListe.setBorder(new LineBorder(Color.black));
		laListe.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);
		laListe.addListSelectionListener(new ListSelectionListener() {
			public void valueChanged(ListSelectionEvent e) {
				if (laListe.isSelectionEmpty() == false) {
					File f1 = (File) laListe.getSelectedValue();
					SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy H:mm:ss");
					Date d = new Date(f1.lastModified());
					nomTexte.setText(f1.getName());
					dateTexte.setText(sdf.format(d));
					tailleTexte.setText(getFormatedSize(f1));
				}
			}
		});

		panel.updateUI();
	}

	public String getFormatedSize(File file) {
		int size = (int) (file.length() / 1024) + 1;
		if (size > 1024) {
			return (size / 1024) + " Mo";
		} else {
			return size + " ko";
		}
	}
	
	private JMenuBar creationBarreDeMenu() {
		fichier.add(itemNouveau);
		fichier.add(itemSauver);
		fichier.add(itemCharger);
		fichier.add(itemQuitter);

		aide.add(itemPropos);
		aide.add(itemAide);

		jmb.add(fichier);
		jmb.add(aide);

		itemQuitter.addActionListener(new Quitter());
		itemNouveau.addActionListener(new Nouveau());
		itemSauver.addActionListener(new Sauvegarder());
		itemCharger.addActionListener(new Charger());
		itemPropos.addActionListener(new Propos());
		itemAide.addActionListener(new Aide());

		return jmb;
	}

	private class TriParNom implements ActionListener {
		public void actionPerformed(ActionEvent arg0) {
			laListe.setListData(a1.getListe().toArray());
		}
	}

	private class RechercheFichier implements KeyListener {
		public void keyPressed(KeyEvent e) throws NoSuchElementException {
			if (e.getKeyCode() == KeyEvent.VK_ENTER) {
				String str = rechercheTexte.getText();
				try {
					if (str.equals(a1.rechercherFichier(str).getName())) {
						SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy H:mm:ss");
						Date d = new Date(a1.rechercherFichier(str).lastModified());
						nomTexte.setText(a1.rechercherFichier(str).getName());
						dateTexte.setText(sdf.format(d));
						tailleTexte.setText(getFormatedSize(a1.rechercherFichier(str)));
					}
				} catch (NoSuchElementException f) {
					System.err.println(f.getMessage());
					new Erreur();
				}
			}
		}

		@Override
		public void keyReleased(KeyEvent arg0) {
			// TODO Auto-generated method stub

		}

		@Override
		public void keyTyped(KeyEvent arg0) {
			// TODO Auto-generated method stub

		}

	}

	public class ClicGauche implements MouseListener{

		@Override
		public void mouseClicked(MouseEvent e) {
			// TODO Auto-generated method stub
			
		}

		@Override
		public void mouseEntered(MouseEvent e) {
			// TODO Auto-generated method stub
			
		}

		@Override
		public void mouseExited(MouseEvent e) {
			// TODO Auto-generated method stub
			
		}

		@Override
		public void mousePressed(MouseEvent e) {
			if (e.getButton() == MouseEvent.BUTTON1)//Bouton droit
			{
				rechercheTexte.setText("");
			}
		}

		@Override
		public void mouseReleased(MouseEvent e) {
			// TODO Auto-generated method stub
			
		}
		
	}
	
	private class TriParTaille implements ActionListener {
		public void actionPerformed(ActionEvent arg0) {
			laListe.setListData(at1.getListe().toArray());
		}
	}

	private class Quitter implements ActionListener {
		public void actionPerformed(ActionEvent e) {
			String action = e.getActionCommand();
			if (action.equals("Quitter")) {
				System.exit(0);
			}
		}
	}

	private class Nouveau implements ActionListener {
		public void actionPerformed(ActionEvent e) {
			String action = e.getActionCommand();
			if (action.equals("Nouveau")) {
				laListe.setListData(liste.toArray());
				nomTexte.setText("");
				dateTexte.setText("");
				tailleTexte.setText("");
				rechercheTexte.setText("");
				laListe.setSelectedIndex(-1);
			}
		}
	}

	private class Sauvegarder implements ActionListener {
		public void actionPerformed(ActionEvent e) {
			String action = e.getActionCommand();
			if (action.equals("Sauvegarder")) {
				a1.serialize("test");
			}
		}
	}

	private class Charger implements ActionListener {
		public void actionPerformed(ActionEvent e) {
			String action = e.getActionCommand();
			if (action.equals("Charger")) {
				a1.read("test");
				laListe.setListData(a1.getListe().toArray());
			}
		}
	}

	private class Propos implements ActionListener {
		public void actionPerformed(ActionEvent e) {
			String action = e.getActionCommand();
			if (action.equals("A Propos")) {
				new APropos();
			}
		}
	}

	private class Aide implements ActionListener {
		public void actionPerformed(ActionEvent e) {
			String action = e.getActionCommand();
			if (action.equals("Aide")) {
				new Aider();
			}
		}
	}



	public static void main(String... args) {
		new ArboIHM();
	}

	@Override
	public void actionPerformed(ActionEvent arg0) {
		// TODO Auto-generated method stub
		
	}
}

package projet;

import java.io.File;
import java.util.List;
import java.util.NoSuchElementException;

public interface ArboTriee {

	public File rechercherFichier(String nom) throws NoSuchElementException;
	public void parcourir(File file);
	public List<File> getListe();
	public void setListe(List<File> liste);
	public void serialize(String fileName);
	public void read(String fileName) ;
	public String toString();
	
}

package projet;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.Date;
import java.util.List;
import java.util.NoSuchElementException;

import javax.swing.JFileChooser;

public class ArboTrieeParTaille implements Comparator<File>, ArboTriee {
	private List<File> liste = new ArrayList<File>();
	private File file;

	public ArboTrieeParTaille(File file) {
		parcourir(file);
		Collections.sort(liste, new MonComparatorTaille());
	}

	public void parcourir(File file) {
		if (file.isDirectory()) {
			File[] list = file.listFiles();
			if (list != null) {
				for (int i = 0; i < list.length; i++) {
					parcourir(list[i]);
				}
			}
		} else {
			if (file.getName().endsWith("pdf")) {
				liste.add(file);
			}
		}
	}

	public File rechercherFichier(String nom) throws NoSuchElementException {
		for (File file : liste) {
			if (file.getName().equals(nom)) {
				return file;
			}
		}
		throw new NoSuchElementException(
				"Il n'y a pas de fichier ayant le nom : " + nom);

	}

	public List<File> getListe() {
		return liste;
	}

	public void setListe(List<File> liste) {
		this.liste = liste;
	}

	public Long getSize() {
		return this.file.length();
	}

	public int compare(File o1, File o2) {
		long nb1 = o1.length();
		long nb2 = o2.length();
		if (nb1 > nb2) {
			return 1;
		} else {
			if (nb1 == nb2) {
				return 0;
			} else {
				return -1;
			}
		}
	}

	public String getFormatedSize(File file) {
		int size = (int) (file.length() / 1024) + 1;
		if (size > 1024) {
			return (size / 1024) + " Mo";
		} else {
			return size + " ko";
		}
	}

	public static String getFileExt(String filename) {
		int pos = filename.lastIndexOf(".");
		if (pos > -1) {
			return filename.substring(pos);
		} else {
			return filename;
		}
	}

    public String getType(File file) {
        JFileChooser chooser = new JFileChooser();
        return chooser.getTypeDescription(file);
    }
	
	public String toString() {
		String s = "Les fichiers pdf suivants sont présents dans l'arborescence : \n";
		for (File fichier : liste) {
			SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy H:mm:ss");
			Date d = new Date(fichier.lastModified());

			s = s + " Nom : " + fichier + " Date : " + sdf.format(d)
					+ " Taille : " + getFormatedSize(fichier)
			 + " Type : " + getType(fichier) + " Extension : "
			 + getFileExt(fichier.getName()) + "\n";
		}
		return s;
	}

	public void serialize(String fileName) {
		ObjectOutputStream oos;
		try {
			oos = new ObjectOutputStream(new FileOutputStream(fileName));
			oos.writeObject(getListe());
			oos.close();
		} catch (FileNotFoundException e) {
			System.err.println(e.getMessage());
		} catch (IOException e) {
			System.err.println(e.getMessage());
		}
		System.out.println("good");
	}

	public void read(String fileName) {
		try {
			ObjectInputStream inputStream = new ObjectInputStream(
					new FileInputStream(fileName));
			List<File> readObject = (List<File>) inputStream.readObject();
			setListe(readObject);
			inputStream.close();
		} catch (FileNotFoundException e) {
			System.err.println(e.getMessage());
		} catch (ClassNotFoundException e) {
			System.err.println(e.getMessage());
		} catch (IOException e) {
			System.err.println(e.getMessage());
		}
	}

}

package projet;

import java.io.*;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Date;
import java.util.List;
import java.util.NoSuchElementException;

import javax.swing.JFileChooser;

public class ArboTrieeParNom implements ArboTriee {

	private List<File> liste = new ArrayList<File>();

	public ArboTrieeParNom(File file) {
		parcourir(file);
		Collections.sort(liste);
	}

	public File rechercherFichier(String nom) throws NoSuchElementException {
		for (File file : liste) {
			if (file.getName().equals(nom)) {
				return file;
			}
		}
		throw new NoSuchElementException(
				"Il n'y a pas de fichier ayant le nom : " + nom);
	}

	public void parcourir(File file) {
		if (file.isDirectory()) {
			File[] list = file.listFiles();
			if (list != null) {
				for (int i = 0; i < list.length; i++) {
					parcourir(list[i]);
				}
			}
		} else {
			if (file.getName().endsWith("pdf")) {
				liste.add(file);
			}
		}
	}

	public List<File> getListe() {
		return liste;
	}

	public void setListe(List<File> liste) {
		this.liste = liste;
	}

	public void serialize(String fileName) {
		ObjectOutputStream oos;
		try {
			oos = new ObjectOutputStream(new FileOutputStream(fileName));
			oos.writeObject(getListe());
			oos.close();
		} catch (FileNotFoundException e) {
			System.err.println(e.getMessage());
		} catch (IOException e) {
			System.err.println(e.getMessage());
		}
		System.out.println("good");
	}

	public void read(String fileName) {
		try {
			ObjectInputStream inputStream = new ObjectInputStream(
					new FileInputStream(fileName));
			List<File> readObject = (List<File>) inputStream.readObject();
			setListe(readObject);
			inputStream.close();
		} catch (FileNotFoundException e) {
			System.err.println(e.getMessage());
		} catch (ClassNotFoundException e) {
			System.err.println(e.getMessage());
		} catch (IOException e) {
			System.err.println(e.getMessage());
		}
	}

	public String getFormatedSize(File file) {
		int size = (int) (file.length() / 1024) + 1;
		if (size > 1024) {
			return (size / 1024) + " Mo";
		} else {
			return size + " ko";
		}
	}

	public static String getFileExt(String filename) {
		int pos = filename.lastIndexOf(".");
		if (pos > -1) {
			return filename.substring(pos);
		} else {
			return filename;
		}
	}

    public String getType(File file) {
        JFileChooser chooser = new JFileChooser();
        return chooser.getTypeDescription(file);
    }
	
	public String toString() {
		String s = "Les fichiers pdf suivants sont présents dans l'arborescence : \n";
		for (File fichier : liste) {
			SimpleDateFormat sdf = new SimpleDateFormat("dd/MM/yyyy H:mm:ss");
			Date d = new Date(fichier.lastModified());

			s = s + " Nom : " + fichier + " Date : " + sdf.format(d)
					+ " Taille : " + getFormatedSize(fichier)
			 + " Type : " + getType(fichier) + " Extension : "
			 + getFileExt(fichier.getName()) + "\n";
		}
		return s;
	}
}

 private class AffichePDF implements ActionListener {  
  public void actionPerformed(ActionEvent arg0) {  
   if ( Desktop.isDesktopSupported() ) { 
    Desktop desktop = Desktop.getDesktop(); 
    if (desktop.isSupported(Desktop.Action.OPEN)) { 
     try { 
      desktop.open(f1); 
     } catch (IOException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 
    } 
   } 
  }  
  }

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException: File must not be null
	at java.awt.Desktop.checkFileValidation(Unknown Source)
	at java.awt.Desktop.open(Unknown Source)
	at projet.ArboIHM$AffichePDF.actionPerformed(ArboIHM.java:284)
	at javax.swing.AbstractButton.fireActionPerformed(Unknown Source)
	at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source)
	at javax.swing.DefaultButtonModel.setPressed(Unknown Source)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source)
	at java.awt.Component.processMouseEvent(Unknown Source)
	at javax.swing.JComponent.processMouseEvent(Unknown Source)
	at java.awt.Component.processEvent(Unknown Source)
	at java.awt.Container.processEvent(Unknown Source)
	at java.awt.Component.dispatchEventImpl(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
	at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
	at java.awt.Container.dispatchEventImpl(Unknown Source)
	at java.awt.Window.dispatchEventImpl(Unknown Source)
	at java.awt.Component.dispatchEvent(Unknown Source)
	at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
	at java.awt.EventQueue.access$000(Unknown Source)
	at java.awt.EventQueue$1.run(Unknown Source)
	at java.awt.EventQueue$1.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue$2.run(Unknown Source)
	at java.awt.EventQueue$2.run(Unknown Source)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(Unknown Source)
	at java.awt.EventQueue.dispatchEvent(Unknown Source)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
	at java.awt.EventDispatchThread.run(Unknown Source)