Not serializable class with strings only

Question

I have a class called Libro which contains strings only:

public class Libro {

    private String titolo;
    private String autore;
    private String editore;
    private String sottotitolo;
    private String genere;
    private String dpubb;
    private String lpubb;
    private String soggetto;
    private String isbn;
    private String note;
    private String prezzo;
    private String npag;

    //getters, setters, constructors...
}

Then I created an object called a belonging to the Libro class, and I tried to write it to a coso.dat file

try{
    ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream("C:\\Users\\Workbench\\Documents\\NetBeansProjects\\coso.dat"));
    out.writeObject(a);
    out.close();
}catch(Exception e) {System.out.println("Damn");}

Then I opened the file with NotePad and it seems that I tried to serialize an unserializable class:

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detailMessaget Ljava/lang/String;[ 
stackTracet [Ljava/lang/StackTraceElement;L suppressedExceptionst Ljava/util/List;xpq ~     t ginobook.classi.Librour [Ljava.lang.StackTraceElement;F*<<ý"9  xp   )sr java.lang.StackTraceElementa  Åš&6Ý… I 
lineNumberL declaringClassq ~ L fileNameq ~ L 
methodNameq ~ xp   t java.io.ObjectOutputStreamt ObjectOutputStream.javat writeObject0sq ~ 
  \q ~ q ~ t writeObjectsq ~ 
  ôt ginobook.forms.FormNuovot FormNuovo.javat btnCreaActionPerformedsq ~ 
   q ~ q ~ t access$1500sq ~ 
  Œt ginobook.forms.FormNuovo$15q ~ t actionPerformedsq ~ 
  æt javax.swing.AbstractButtont AbstractButton.javat fireActionPerformedsq ~ 
    ,t "javax.swing.AbstractButton$Handlerq ~ q ~ sq ~ 
  ’t javax.swing.DefaultButtonModelt DefaultButtonModel.javaq ~  sq ~ 
  q ~ $q ~ %t 
setPressedsq ~ 
   üt *javax.swing.plaf.basic.BasicButtonListenert BasicButtonListener.javat 
mouseReleasedsq ~ 
  …t java.awt.Componentt Component.javat processMouseEventsq ~ 
  üt javax.swing.JComponentt JComponent.javaq ~ /sq ~ 
  šq ~ -q ~ .t processEventsq ~ 
  ¼t java.awt.Containert Container.javaq ~ 4sq ~ 
  q ~ -q ~ .t dispatchEventImplsq ~ 
  öq ~ 6q ~ 7q ~ 9sq ~ 
  gq ~ -q ~ .t 
dispatchEventsq ~ 
  t java.awt.LightweightDispatcherq ~ 7t retargetMouseEventsq ~ 
  ­q ~ >q ~ 7q ~ /sq ~ 
  rq ~ >q ~ 7q ~ <sq ~ 
  èq ~ 6q ~ 7q ~ 9sq ~ 

ºt java.awt.Windowt Window.javaq ~ 9sq ~ 
  gq ~ -q ~ .q ~ <sq ~ 
  öt java.awt.EventQueuet EventQueue.javaq ~ 9sq ~ 
   aq ~ Hq ~ It 
access$500sq ~ 
  Åt java.awt.EventQueue$3q ~ It runsq ~ 
  ¿q ~ Mq ~ Iq ~ Nsq ~ 
ÿÿÿþt java.security.AccessControllert AccessController.javat doPrivilegedsq ~ 
   Lt 5java.security.ProtectionDomain$JavaSecurityAccessImplt ProtectionDomain.javat doIntersectionPrivilegesq ~ 
   Vq ~ Uq ~ Vq ~ Wsq ~ 
  Ût java.awt.EventQueue$4q ~ Iq ~ Nsq ~ 
  Ùq ~ Zq ~ Iq ~ Nsq ~ 
ÿÿÿþq ~ Qq ~ Rq ~ Ssq ~ 
   Lq ~ Uq ~ Vq ~ Wsq ~ 
  Øq ~ Hq ~ Iq ~ <sq ~ 
   Ét java.awt.EventDispatchThreadt EventDispatchThread.javat pumpOneEventForFilterssq ~ 
   tq ~ `q ~ at pumpEventsForFiltersq ~ 
   iq ~ `q ~ at pumpEventsForHierarchysq ~ 
   eq ~ `q ~ at 
pumpEventssq ~ 
   ]q ~ `q ~ aq ~ hsq ~ 
   Rq ~ `q ~ aq ~ Nsr &java.util.Collections$UnmodifiableListü%1µìŽ L listq ~ xr ,java.util.Collections$UnmodifiableCollectionB €Ë^÷ L ct Ljava/util/Collection;xpsr java.util.ArrayListxÒ™Ça I sizexp    w    xq ~ px

What did I do wrong..? I'm sorry if the question may seem stupid, but I just couldn't find any answer to it