probleme en java

import javax.swing.*;

public class fenetre {

public static void main(String[] args) {

fen fenetre=new fen(400,150,"Test" );
}
}

class fen extends JFrame {

private int hauteur;
private int largeur;
private String nom;

fen(int l,int h,String n) {

largeur=l;
hauteur=h;
nom=n;
setTitle(nom);
setSize(l,h);
}

}

1. WARNING in fenetre.java
(at line 15)
class fen extends JFrame {
^^^
The serializable class fen does not declare a static final serialVersionUID field of type long