Ajouter noeuds dans Jtree définitivement

import java.awt.event.*;
import javax.swing.*;
import javax.swing.tree.*;
import java.io.*;

public class SauverJTree extends JFrame {
	
	JTree arbre;
	DefaultMutableTreeNode racine;
	
	String chemin = "Exemple.dat";
	
	public SauverJTree(){
		
		setTitle("Exemple JTree");
		setSize(300, 500);
		
		racine = new DefaultMutableTreeNode("Racine");
		DefaultMutableTreeNode n1 = new DefaultMutableTreeNode("Un noeud");
		DefaultMutableTreeNode n2 = new DefaultMutableTreeNode("Un autre noeud");
		DefaultMutableTreeNode n3 = new DefaultMutableTreeNode("Un troisieme noeud");
		DefaultMutableTreeNode n4 = new DefaultMutableTreeNode("Un dernier");
		
		n1.add(n2);
		n1.add(n3);
		racine.add(n1);
		racine.add(n4);
		
		arbre = new JTree(racine);
		JScrollPane scroll = new JScrollPane(arbre);
		
		getContentPane().add(scroll);
		
		addWindowListener(new WindowAdapter(){
			public void windowClosing(WindowEvent e){
				System.out.println("Saving");
				
				try{    
            
					FileOutputStream fichier = new FileOutputStream(chemin); 
					ObjectOutputStream out = new ObjectOutputStream(fichier); 
              
            
					out.writeObject(racine); 
              
					out.close(); 
					fichier.close(); 
					
				}catch(IOException ex){ 
					ex.printStackTrace(); 
				} 
				
				System.exit(0);
			}
		});
		
	}
		
	public static void main (String args[]) {
		(new SauverJTree()).setVisible(true);
	}
}

import java.awt.event.*;
import javax.swing.*;
import javax.swing.tree.*;
import java.io.*;

public class ChargerJTree extends JFrame {
	
	JTree arbre;
	DefaultMutableTreeNode racine;
	
	String chemin = "Exemple.dat";
	
	public ChargerJTree(){
		
		setTitle("Exemple JTree");
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setSize(300, 500);
		
		try{     
			FileInputStream fichier = new FileInputStream(chemin); 
			 ObjectInputStream in = new ObjectInputStream(fichier); 
              
			 racine = (DefaultMutableTreeNode)in.readObject(); 
              
			 in.close(); 
			fichier.close(); 
              
		 }catch(Exception ex){ 
			  ex.printStackTrace(); 
		 } 
		
		if(racine == null){
			racine = new DefaultMutableTreeNode("Vide");
		}
		
		arbre = new JTree(racine);
		JScrollPane scroll = new JScrollPane(arbre);
		
		getContentPane().add(scroll);
		
	}
	
	public static void main (String args[]) {
		(new ChargerJTree()).setVisible(true);
	}
}