Re: Read binary data file

From:
 Hunter Gratzner <a24900@googlemail.com>
Newsgroups:
comp.lang.java.programmer
Date:
Wed, 29 Aug 2007 14:06:09 -0700
Message-ID:
<1188421569.371760.28450@22g2000hsm.googlegroups.com>
On Aug 29, 9:52 pm, Windsor.Lo...@gmail.com wrote:

I am a C++ programmer, working on a java program. I need to read a
binary file using Java.

Here is how I read it in C++,

Struct SOME_DATA
{
unsigned long data1;
unsigned short data2;
unsigned short data3;
unsigned long data4;

}

struct SOME_DATA someData;

and read using

fread(&someData, 12, 1, inputFile);


This is already a stupid idea in C++, since there is no guarantee that
sizeof(SOME_DATA) == 12. Since this is a Java group I'd like to
recommend that you consult some C++ resource regarding struct
alignment and padding, data type size, and (network) byte order.

In Java (assuming you have fixed you C++ problem), one would read this
e.g. with a DataInputStream:

/*
 * Read data using network byte-order, aka big-endian
 * byte-order (MSB first), and no padding/alignment
 * between the data.
 */
class Data {
   /*
    * Note, Java has no unsigned data types.
    * Therefore in this example I store the unsigned short
    * in a (signed) int, and the unsigned long in a BigInteger
    * Typically, in a carefully designed application this
    * can be avoided, but I do it here to avoid discussion
    * of using signed data types to handle unsigned types.
    */
   private BigInteger data1; // data format: unsigned long64
   private int data2; // data format: unsigned short16
   private int data3; // data format: unsigned short16
   private BigInteger data4; // data format: unsigned long64

   public void read(DataInputStream in) throws IOException {
      byte ulong2big[] = new byte[5];
      ulong2big[0] = 0; // ensure MSB is always zero, so
                         // we get an unsigned interpretation
                         // of the following 4 byte data
                         // when converting the array to a
                         // BigInteger

      // Read four bytes and convert them to a BigInteger
      // In carefully designed applications a
      // data1 = in.readLong() would do.
      in.read(ulong2big, 1, 4); // TODO: check return value
      data1 = new BigInteger(ulong2big);

      // Read the unsigned short into an int
      data2 = in.readUnisgnedShort();
      // in.skipByte(...) in case padding needs to be skipped

      data3 = in.readUnsignedShort();
      // in.skipByte(...) in case padding needs to be skipped

      // Read four bytes and convert them to a BigInteger
      // In carefully designed applications a
      // data4 = in.readLong() would do.
      in.read(ulong2big, 1, 4); // TODO: check return value
      data4 = new BigInteger(ulong2big);
  }
}

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