How to read multiple objects stored in a file.

Hi,
there is a file on which multiple object have been written.
Now while reading it back, is it necessary to read the objects in the same sequence in which they were written or i can randomly read any object?
thanx
deepak saini

I suppose you could use random file access, but you would need to keep track of where you put the objects.

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How to multiple objects stored in a file.

Hi,
there is a file on which multiple object have been written.
Now while reading it back, is it necessary to read the objects in the same sequence in which they were written or i can randomly read any object?
thanx
deepak saini

If that's really a concern, you could write the objects to separate files.
Alternatively, if you're really keen, look at RandomAccessFile. You'd have to build InputStream and OutputStream bridges to be able to use ObjectInputStream and ObjectOutputStream. You'd probably want to use BufferedInputStream and BufferedOutputStream in there too.
You'd start by writing a long of value zero the file. Then use a new ObjectOuptutStream for each object (rembering to close each one), retaining a List of offsets into the file where the objects start. Then you'd write the List itself using another ObjectOutputStream. Finally you position back to the beginning of the file, and write the offset of the List as a long, overwriting the long zero value.
The reading side is left for as exercise for the reader.
This would allow you to read directly any object given its index into the List.
This is only worthwhile if you're talking about lots of objects.
And, as I said, only if you're keen :)
Sylvia.

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Hi,
try this
CREATE TABLE ab
a VARCHAR2(1 BYTE),
b VARCHAR2(2 BYTE),
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d VARCHAR2(11 BYTE)
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( TYPE ORACLE_LOADER
DEFAULT DIRECTORY dir_name
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fields (
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b POSITION(*) CHAR(2 ),
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d POSITION(*) CHAR(11 )
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REJECT LIMIT UNLIMITED
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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");

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.

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AK
<pre style="font-family: 'Monaco', 'Courier New', Courier, monospace; overflow:auto; color: #222; background: #DDD; padding: 0.2em; font-size: 10px; width:400px">on run
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set TheFile to open for access (path to desktop as text) & "temp.txt"
set TheContents to read TheFile --until return
close TheFile
set TheLines to paragraphs of TheContents
set OldDelim to AppleScript's text item delimiters
set AppleScript's text item delimiters to {"|"}
set LCount to 0
set ACount to 0
set RCount to 0
set DCount to 0
set SCount to 0
set HowMany to (count of TheLines)
try
repeat with ThisLine in TheLines
if (count of ThisLine) is 0 then exit repeat
-- Start Date, Title, End Date, Calendar, All Day, Recurs, Detached
set LCount to LCount + 1
set Details to text items of ThisLine
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set MyTitle to item 2 of Details
set MyEndDate to FixMyDate(item 3 of Details)
set MyCalendar to item 4 of Details
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set MyDetached to item 7 of Details
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if (count of MyRecurs) > 0 then set RCount to RCount + 1
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tell calendar MyCalendar
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tell ThisItem
if MyAllDay is "1" then set allday event to true
if (count of MyRecurs) > 0 then set recurrence to MyRecurs
set end date to MyEndDate
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end tell
else
set SCount to SCount + 1
end if
if (LCount mod 50) = 0 then
set the_message to "Processed " & (LCount as string) & " of " & (HowMany as string)
display dialog the_message buttons {"Cancel"} giving up after 1
end if
end repeat
on error TheError
display dialog "Error: " & TheError & " about line " & LCount
end try
set AppleScript's text item delimiters to OldDelim
display dialog (LCount as string) & " lines processed" & return & "All Day: " & (ACount as string) & return & "Recurs: " & (RCount as string) & return & "Detach: " & (DCount as string) & return & "Skipped: " & (SCount as string)
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on FixMyDate(MyDate)
date (do shell script "date -r " & MyDate & " -v+31y +%e'/'%m'/'%y' '%T")
end FixMyDate
</pre>

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How to read appended objects from file with ObjectInputStream? The short answer is you can't.
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How to read multiple objects stored in a file.

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