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Reading C++ created binary file in Java...

Posted on 2004-04-14
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Last Modified: 2008-02-26
For my Networking class, my partner and I have been assigned the task of creating a client/server application.  The details of the project are thus:

-Our instructor wrote a small C++ program that writes flight data in a binary file.  His program takes an ascii file of the data, and writes a binary version.

Code for program that creates the binary file:  http://www.atthefrontsystems.com/scheduler.c
Header for the flitdata structure:  http://www.atthefrontsystems.com/sched.h
ASCII sample datafile: http://www.atthefrontsystems.com/sched.ascii
Binary sample datafile: http://www.atthefrontsystems.com/sched.db

-Our task is to create a Java server and client app.  The client will pass fairly simple queries to the server (i.e. list all flights out of a certain airport).    The server will then return the results to the client, who displays the data on the screen.

Details on project: http://www.atthefrontsystems.com/project.pdf

My problem is this.  I can't seem to properly read the binary data.  I'm still a novice programmer in both Java and C++, but it seems to me that the C++ program just wrote most of the file as it's primitive data types.  Taking this into account, I tried to read it in using the "equivalent" Java data types.  For instance, the beginning of the datafile is the Flight#, written (seemingly) as a C-string.  So to test my theroy, I wrote some small code to just read the first char, and print it:

----
import java.io.*;

public class readit {
      public static void main (String args[]) throws Exception {
            FileInputStream fis = new FileInputStream("sched.db");
            DataInputStream dis = new DataInputStream(fis);

            System.out.println(dis.readChar());
      }
}
----

Instead of printing the expected 'A', I get a '?'

Undoubtdly, I'm going about this completely wrong, and would really appreciate some help with this problem.  Should I be RandomAccessing the file instead of using a Stream?  Is there some trick to manual type conversion that I'm overlooking?

If someone could please look through the linked material contained in this rambling post, and "show me the way", I'd be forever in their debt.

P.S.  (It's Friday, April 23d... so I'm not waiting till the COMPLETELY last minute.)  ;)
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Question by:MTXperience
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Expert Comment

by:CEHJ
ID: 10828508
>>Instead of printing the expected 'A', I get a '?'

It's probably not a char (two-byte value) in the sense of a letter code and therefore won't decode properly. If it's a two-byte number then you could do:

System.out.println((int)dis.readChar());
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Accepted Solution

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CEHJ earned 130 total points
ID: 10828527
In point of fact, it looks like the flight number is four bytes. You could therefore do

byte[] flightNum = new byte[4];
dis.read(flightNum);
System.out.println(new String(flightNum));

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Expert Comment

by:objects
ID: 10828584
how to read it is a little system dependant.
also C strings are zero terminated, whereas Java strings are not.
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Expert Comment

by:objects
ID: 10828617
why exactly does your Java server need to use the binary file created by the C program.
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Author Comment

by:MTXperience
ID: 10829057
CEHJ, your code for reading the chars worked splendidly.  Any insight into how to read the ints?  (3 of them... 2 in the Time structure... 1 other)  How about the float?
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Author Comment

by:MTXperience
ID: 10829067
>>why exactly does your Java server need to use the binary file created by the C program.

Answer:  Because our professor told us too.
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Expert Comment

by:objects
ID: 10829074
try readInt() and readFloat()
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Expert Comment

by:objects
ID: 10829076
> Because our professor told us too.

they are annoying like that aren't they :)

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Expert Comment

by:CEHJ
ID: 10829080
You can read various primitives with DataInputStream. See

http://java.sun.com/j2se/1.5.0/docs/api/java/io/DataInputStream.html
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Expert Comment

by:objects
ID: 10829083
C data types can vary in size though, so you'll need to determine exactly what size is being used.
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Author Comment

by:MTXperience
ID: 10829185
Thanks guys, we knew about the primitive data type reading... we just made a small oversight in our tests that skewed things at first.  But that all seems to be working fine now.  Again... thanks much for your help.
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Expert Comment

by:CEHJ
ID: 10829242
:-)

You seem to be getting on fine, but you should probably create a class of type, say, FlightInformation and read the binary file such that you're left with a collection of FlightInformation objects
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Expert Comment

by:CEHJ
ID: 10829369
>>In point of fact, it looks like the flight number is four bytes.

But is declared as 5 in sched.h
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