Question

I have this program that reads 2 Kb Data from a binary file adds some header to it and then writes it to a new file.
The code is

try {
        FileInputStream fis = new FileInputStream(bin);
        FileOutputStream fos = new FileOutputStream(bin.getName().replace(".bin",    ".xyz"));
        DataOutputStream dos=new DataOutputStream(fos);  
        fos.write(big, 0, big.length);
        for (int n = 1; n <= pcount; n++) {          
            fis.read(file, mark, 2048);
            mark = mark + 2048;             
            prbar.setValue(n);
            prbar.setString("Converted packets:" + String.valueOf(n));
            metas = "2048";
            meta = metas.getBytes();
            pc = String.valueOf(file.length).getBytes();
            nval = String.valueOf(n).getBytes();
            System.arraycopy(pc, 0, bmeta, 0, pc.length);
            System.arraycopy(meta, 0, bmeta, 4, meta.length);
            System.arraycopy(nval, 0, bmeta, 8, nval.length);          
            fos.write(bmeta, 0, bmeta.length);
            fos.flush();
            fos.write(file, 0, 2048);
            fos.flush();
       }
  }catch (Exception ex) {
        erlabel.setText(ex.getMessage());
  }

First it should write the header and then the file.But the output file is full of data that does not belong to the file.It is writing some garbage data.What may be the problem?

Answer 1

It’s not quite clear with some of the declarations missing, but it looks like your problem is with the fis.read() method: the second argument is an offset in the byte array, not the file (common mistake).

You probably want to use relative reads. You also need to check the return value from .read() to see how many bytes were actually read, before writing the buffer out.

The common idiom is:

InputStream is = ... 
OutputStream os = ...

byte[] buf = new byte[2048];
int len;
while((len = is.read(buf)) != -1)
  os.write(buf, 0, len);

is.close();
os.close();

Edit

That’s a pretty weird way of writing out your metadata, I assume that’s what the (unused) DataOutputStream is for?

You don’t need to keep flushing the output stream, just close it when you’re done.