Problems appending to a file

[Frank B. Brokken]

Dear listmembers,

    I have a problem using FileOutputStreams. I want to be able to append to 
an existing file, and I would like to be able to read the information back
later. 

    To write to the file, I use the following small program, in which a
FileOutputStream is opened in append-mode, followed by the writing of two
Strings:

///////////////////////////////////////////////////////////
import java.io.*;

public class writer
{
    public static void main(String argv[])
    {
        try
        {
            FileOutputStream
                f = new FileOutputStream("out", true);  // open in append mode
    
            ObjectOutputStream
                o = new ObjectOutputStream(f);
    
            o.writeObject("hello");                     // write 2 strings
            o.writeObject("world");
            o.flush();
            f.close();
        }
        catch (Exception e)
        {
            System.err.println(e.toString());
        }
    }
}
//////////////////////////////////////////////////////

    Then, to read the file again, I use the following small program:

//////////////////////////////////////////////////////
import java.io.*;

public class reader
{
    public static void main(String argv[])
    {
        try
        {
            FileInputStream
                f = new FileInputStream("out");
    
            ObjectInputStream
                o = new ObjectInputStream(f);
    
            while (true)
                System.err.println("Saw: " + (String)o.readObject());
        }
        catch (Exception e)
        {
            System.err.println(e.toString());
        }
    }
}
////////////////////////////////////////////////////////////

Now, after running writer the first time, 'out' is created, and contains
(hex):

00000000:  AC ED 00 05 74 00 05 68 65 6C 6C 6F 74 00 05 77  ....t..hellot..w
00000010:  6F 72 6C 64                                      orld

The reader program reads this, and shows:

Saw: hello
Saw: world
java.io.EOFException: Expecting code

So far, so good. Now I run writer again, and this results in the following
contents of the file 'out':

00000000:  AC ED 00 05 74 00 05 68 65 6C 6C 6F 74 00 05 77  ....t..hellot..w
00000010:  6F 72 6C 64 AC ED 00 05 74 00 05 68 65 6C 6C 6F  orld....t..hello
00000020:  74 00 05 77 6F 72 6C 64                          t..world

Note the byte at offset 0x14: it is 0xac, which is -84 when the byte is viewed
as a signed 8 bits value. When I now run reader, it tells me:

Saw: hello
Saw: world
java.io.StreamCorruptedException: Type code out of range, is -84

What I make of this is that the initial bytes of the file (ac ed) are
appended every time the writer program is started again. The first time I
start writer the two strings are written consecutively, without any special
bytes in between. So it looks as if the intermediate ac ed bytes shouldn't be
there. 

So, I wonder if I'm doing something wrong here. If any of you guys could help
me out (maybe by sending email directly to [EMAIL PROTECTED]), I'd be very
grateful.

Thanks in advance,

-- 
    Frank B. Brokken        (al sinds enige jaren uitgedost (untranslatable))
    My extended signature is at http://www.icce.rug.nl/frank/signature


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