Writing byte to a particular location in a file header

[john kalaigian]

Ok I have a file with a header which I've read and know the exact location of the particular element that I want to read. What I want to do now is overwrite that particular element of the file with a byte representation of a String. How do i do this, and also without creating a new file, cause I'm using a
FileInStream->BufferedInStream->DataInStream
to do the reading and I don't want to create a brand new
FileOutStream->BufferedOutStream->DataOutStream
to do the writing. I want to just overwrite particular elements in the file header. Thanks, John

[Peter Tran]

The only way to do this is to use java.io.RandomAccessFile and use the "rw" mode.

[john kalaigian]

Ok here's my code if it would help out. I just wanna have you take a look to see if it really has to be done with RandomAccessFile

class Header {
    private boolean littleEndian = true;
    private String fileName;
    private short[] shortBuf;
    Header(String fileName) {
        this.fileName = fileName;
        try {
            FileInputStream fs = new FileInputStream(fileName);
            BufferedInputStream bis = new BufferedInputStream(fs);
            DataInputStream ds = new DataInputStream(bis);
            while(getNextDataElement(ds));
        }
        catch(Exception e) {
            e.printStackTrace();
        }
    }
    
    private boolean getNextDataElement(DataInputStream ds) {
        try {
            int group = ds.readInt();
            int length = ds.readInt();
            short grp = (short)((group >> 16) & 0xffff);
            short elm = (short)(group & 0xffff);
            if(littleEndian) {
                length = swap(length);
                grp = swap(grp);
                elm = swap(elm);
                group = (grp << 16) + elm;
            }
            byte[] buf = new byte[length];
            ds.read(buf, 0, length);
            retrieveData(group, buf);
            return true;
        }
        catch(Exception e) {
            return false;
        }
    }
    
    private static int swap(int i) {
        int j = i >>> 24;
        j |= ((i >>> 16) & 0xff) << 8;
        j |= ((i >>> 8) & 0xff) << 16;
        j |= (i * 0xff) << 24;
        return j;
    }
    
    static short swap(short i) {
        return (short)(((i >>> 8) & 0xff) | ((i & 0xff) << 8));
    }
    
    static short parseShort(byte[] buf) {
        return (short)((buf[0] & 0xff) | ((buf[1] & 0xff) << 8));
    }
    
    double parseDouble(byte[] buf) {
        String s = new String(buf);
        return (new Double(s)).doubleValue();
    }
    
    double[] pareseDoubleArray(byte[] buf) {
        String s = new String(buf);
        double[] darray = new double[(s.length() + 8) / 9];
        int pos;
        int index = 0;
        while((pos = s.indexOf("\\")) >= 0) {
            String sub = s.substring(0, pos-1);
            darray[index++] = (new Double(sub)).doubleValue();
            s = s.substring(pos + 1);
        }
        if(s.length() >=8)
            darray[index++] = (new Double(s)).doubleValue();
        if(index < darray.length) {
            double[] darray1 = new double[index];
            for(int i = 0; i < index; i++)
                darray1[i] = darray[i];
            darray = darray1;
        }
        return darray;
    }
    
    void retrieveData(int group, byte[] buf) {
        switch(group) {
            case 0x00100010: //Header Group 0010, Element 0010 - Written as (0010,0010)
                System.out.println(new String(buf));
                break;
... //Other Cases
            case 0x7fe00010: //Image Data
                shortBuf = new short[buf.length >> 1];
                for(int i = 0; i < buf.length; i += 2) {
                    byte low = buf[i];
                    byte high = buf[i+1];
                    if(high >= 0)
                        shortBuf[i>>1] = (short)((low & 0xff) + ((high & 0xff) <<8));
                    else shortBuf[i>>1] = 0;
                }
        }
    }
    
    public static void main(String[] args) {
        Header dh = new Header(args[0]);
    }
}

What I wanna do is not go into a particular header line say (0020,0010) which is group 0020, and overwrite element 0010, so somewhere in my case statement I will have:

String s = "TEST";
byte[] writeBuf = new byte[length];
writeBuf = s.getBytes();
...
case 0x00200010:
DataOutputStream.write(writeBuf,..., length);
...

Now here's the problem. I don't know where to start writing this data because up in getNextDataElement(DataInStream) I read from the DataInStream starting at 0 and going up to the length provided in the header. Now that I'm sitting in the retrieveData method I'm just parsing through the groups and reading the bytes provided in my byte buffer. Where and how do I write my new String into the file without creating a new file - just modifying the bytes specified in the group. Hope this is a little more comprehensive.
John

[john kalaigian]

Sorry about the above. I wrote:

What I wanna do is not go into a...

It should be

What I wanna do is go into a...

John