Java: multiple outputstreams single file

Question

I need to read multiple small files and append them into a bigger single file.

Base64OutputStream baos = new Base64OutputStream(new FileOutputStream(outputFile, true));

for (String fileLocation : fileLocations) {
InputStream fis = null; 
        try
        {
            fis = new FileInputStream(new File(fileLocation));
            int bytesRead = 0;
            byte[] buf = new byte[65536];
            while ((bytesRead=fis.read(buf)) != -1) {
                if (bytesRead > 0) baos.write(buf, 0, bytesRead);
            }
        }
        catch (Exception e) { 
            logger.error(e.getMessage());
        }
        finally{
            try{
                if(fis != null)
                    fis.close();
            }
            catch(Exception e){
                logger.error(e.getMessage());
            }
        }
}

All pretty standard, but I'm finding that, unless I open a new baos per input file (include it inside the loop), all the files following the first one written by baos are wrong (incorrect output).

The questions:

• I've been told that opening/closing an outputstream back and forth for the same resource is not a good practice, why?
• Why using a single output stream is not delivering the same result as multiple separate ones?

Answer 1

Perhaps the problem is that if you are assumming that encoding in base64 the concatenation of several files should give the same result as concatenating the base64 encoding of each file? That's not necessariy the case; base64 encodes groups of three consecutive input bytes to 4 ascii characters, so, unless you know that each file has a size that is a multiple of three, the base64 encoding will produce completely different outputs.