Object Serialization

[Julien Castelain]

hi all,

i'm trying acheive a simple task :

1 - Create one (or many) custom object(s)
2 - Save them to a file
3 - Read the objects from the file

1 - i created a class called "Client" that implements Serializable

2 - here's the way i thought of saving these object :
use an ArrayList (clients) to store the objects
every time a new "Client" object is created, add it
to the ArrayList...
when it comes to "saving" the objects

    public void saveClients()
    {
        try
        {
            ObjectOutputStream os = new ObjectOutputStream(
                    new FileOutputStream(mainFile));
           
            int len = clients.size ();
            for(int i = 0; i < len ; i++)
            {
                Client c = clients.get(i);
                os.writeObject (c);
System.out.println("Writing object : " + c);
            }
            os.close();
        }
        catch(Exception ex) { ex.printStackTrace (); }
    }
    

3 - this seems to work so i'll try to read the objects

    public void readClients()
    {
        try 
        {
            ObjectInputStream is = new ObjectInputStream(
                    new FileInputStream(mainFile));
            
            Client obj = null ;

    while((obj = (Client) is.readObject()) != null)
            {
       System.out.println("readClients :" + obj);
            }
        }
        catch(Exception ex) { ex.printStackTrace (); }
    }
    

i get an Exception (i guess i have reached the end of the file)
, but how do i know when i have to stop "reading" ?

thanks for your help

[Jim Yingst]

Unfortunately serialization does not offer an elegant way to detect the last object without throwing an error, once they have been serialized this way. You have several alternatives:

Serialize the entire ArrayList at once. This will serialize the contents as well. (You could also serialize an array instead.)

Use writeInt() to write the number of objects in the list, [i]before[i] you write all the individual objects. Then before deserializing use readInt() to read the number of objects, then loop to read only that number of objects.

Create a special object which signals the end of input.

[Julien Castelain]

Thanks Jim,

I changed the two methods for writing / reading

 
    public void saveClients()
    {
        try
        {
            ObjectOutputStream os = new ObjectOutputStream(
                    new FileOutputStream(mainFile));
            int len = clients.size ();
            os.writeInt(len);
    for(int i = 0; i < len ; i++)
            {
                Client c = clients.get(i);
                os.writeObject (c);
System.out.println("Writing object : " + c);
            }
            os.close();
        }
        catch(Exception ex) { ex.printStackTrace (); }
    }
//
     public void readClients()
    {
        try 
        {
            ObjectInputStream is = new ObjectInputStream(
                    new FileInputStream(mainFile));
            
            Client obj = null ;

    /*
    while((obj = (Client) is.readObject()) != null)
            {
System.out.println("readClients :" + obj);
            }
    */
    int len = is.readInt();
    for (int i = 0; i < len; i++)
    {
obj = (Client) is.readObject() ;
System.out.println("readClients : " + obj.getName());
    }
        }
        catch(Exception ex) { ex.printStackTrace (); }
    }

no Exception this time