Java how to read a binary file prepared by a c program

A. The bits of the primitives are different in c and in Java.

Java:
       byte:              8 bits/1 byte
       short/char:     16 bits/2 bytes
       int/float:         32 bits/4 bytes
       long/double:   64 bits/8 bytes

c:
       char:              8 bits/1 byte
       short:             16 bits/2 bytes
       int/long/float:  32 bits/4 bytes
       double:          64 bits/8 bytes

B. The byte order in Unix and Windows are different. In general Unix uses the big endian and windows uses the little endian.

     

C. If a binary file is prepared by a c program and you are reading it in a java program, you need to adjust the read method to ensure that you read the right amount of bytes of that data type in c. If the binary file is prepared on a Unix machine and is read in a Windows system or vise versa, you need to adjust the byte order accordingly.

       File theFile = new File("<fileName>");
       FileInputStream fis = new FileInputStream(theFile);

       byte[] byteArray = new byte[(int)theFile.length()];

       fis.read(byteArray);

       ByteBuffer buffer = ByteBuffer.wrap(byteArray);


       //Add the following line if the binary file is prepared on a Unix machine and read on a Windows machine
       buffer.order(ByteOrder.LITTLE_ENDIAN);
       //Add this line if it is prepared on a Windows system and read on a Unix machine
       buffer.order(ByteOrder.BIG_ENDIAN);

 To read a char, instead of using buffer.getChar() which reads two bytes in java, you need to use (char)buffer.get(). 

To read a long, instead of using buffer.getLong() which reads 8 bytes in java, you need to use (new Integer(buffer.readInt())).longValue().

References:

1.Java read from and write to a binary stream

2. How to convert byte array or char array to string in Java

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