how to increase a number with serialization

I want to increase a number with serialization but I can't do it.

[THIS IS THE COMPLETE CODE]

System Class

package ejemplo;

class Sistema{

    public static void main(String[] args) {
        /*Num numero = new Num(0);
        Serializadora serializadora = new Serializadora();
        serializadora.escribiendo(numero);*/
        Indice indice = null;
        Serializadora serializadora = new Serializadora();
        numero = (Num) serializadora.leerObjeto("objeto.dat");
        System.out.println(numero.getF());
    }
}

Class Number

package ejemplo;

import java.io.Serializable;

import javax.swing.text.StyledEditorKit.ForegroundAction;

public class Num implements Serializable {

    private int f;

    public int getF() {
        return f;
    }

    public void setF(int f) {
        this.f=f;
    }

    public void postSerializacion(){
        if (f<20){
            f++;
        }
        else {
            f=0;
        }
        System.out.println("este es el numero "+f);
    }


    public Num(int f) {
        super();
        this.f = f;
        System.out.println(f);
    }
}

Serializer class

package ejemplo;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

public class Serializadora {
    private ObjectInputStream lee;
    private ObjectOutputStream escribe;

    public void escribiendo(Object objeto) {
        try {
            escribe = new ObjectOutputStream(new FileOutputStream("objeto.dat"));
            escribe.writeObject(objeto);
        } catch (FileNotFoundException e){
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public Object leerObjeto(String nombredelarchivo) {
        Object retorno = null;
        try {
            lee = new ObjectInputStream(new FileInputStream(nombredelarchivo));
            retorno = lee.readObject();
            if(retorno instanceof Num){
                ((Num)retorno).postSerializacion();
            }
        } catch (FileNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return retorno;
    }
}

What I want to do is if f is less than 20, increase the serialized number by one by replacing f until reaching f=20 and at the end print "this is the number {here the number}"