How to read appended objects from file with ObjectInputStream?

Question

Hi to everyone. I'm new to Java so my question may look really stupid to most of you but I couldn't fined a solution by myself... I wanted to make an application, something like address book that is storing information about different people. So I decided to make a class that will hold the information for each person (for example: nickname, name, e-mail, web address and so on), then using the ObjectOutputStream the information will be save to a file. If I want to add a new record for a new person I'll simply append it to the already existing file. So far so good but soon I discovered that I can not read the appended objects using ObjectInputStream.
What I mean is that if I create new file and then in one session save several objects to it using ObjectOutputStream they all will be read with no problem by ObjectInputStream. But after that if in a new session I append new objects they won't be read. The ObjectInputStream will read the objects from the first session after that IOException will be generated and the reading will stop just before the appended objects from the second session.
The following is just a simple test it's not actual code from the program I was talking about. Instead of objects containing different kind of information I'm using only strings here. To use the program use as arguments in the console "w" to create new file followed by the file name and the strings you want save to the file (as objects). Example: "+w TestFile.obj Thats Just A Test+". Then to read it use "r" (for reading), followed by the file name. Example "+r TestFile.obj+". As a result you'll see that all the strings that are saved in the file can be successfully read back. Then do the same: "+w TestFile.obj Thats Second Test+" and then read again "+r TestFile.obj+". What will happen is that the strings only from the first sessions will be read and the ones from the second session will not.
I am sorry for making this that long but I couldn't explain it more simple. If someone can give me a solution I'll be happy to hear it! ^.^ I'll also be glad if someone propose different approach of the problem! Here is the code:
import java.io.*;
class Fio
     public static void main(String[] args)
          try
               if (args[0].equals("w"))
                    FileOutputStream fos = new FileOutputStream(args[1], true);
                    ObjectOutputStream oos = new ObjectOutputStream(fos);
                    for (int i = 2; i < args.length ; i++)
                         oos.writeObject(args);
                    fos.close();
               else if (args[0].equals("r"))
                    FileInputStream fis = new FileInputStream(args[1]);
                    ObjectInputStream ois = new ObjectInputStream(fis);
                    for (int i = 0; i < fis.available(); i++)
                         System.out.println((String)ois.readObject());
                    fis.close();
               else
                    System.out.println("Wrong args!");
          catch (IndexOutOfBoundsException exc)
               System.out.println("You must use \"w\" or \"r\" followed by the file name as args!");
          catch (IOException exc)
               System.out.println("I/O exception appeard!");
          catch (ClassNotFoundException exc)
               System.out.println("Can not find the needed class");

Answer 1

How to read appended objects from file with ObjectInputStream? The short answer is you can't.
The long answer is you can if you put some work into it. The general outline would be to create a file with a format that will allow the storage of multiple streams within it. If you use a RandomAccessFile, you can create a header containing the length. If you use streams, you'll have to use a block protocol. The reason for this is that I don't think ObjectInputStream is guaranteed to read the same number of bytes ObjectOutputStream writes to it (e.g., it could skip ending padding or such).
Next, you'll need to create an object that can return more InputStream objects, one per stream written to the file.
Not trivial, but that's how you'd do it.