Good design: How to pass InputStreams as argument?

Question

I've got a big file on which I'm opening a FileInputStream. This file contains some files each having an offset from the beginning and a size. Furthermore, I've got a parser that should evaluate such a contained file.

File file = ...; // the big file
long offset = 1734; // a contained file's offset
long size = 256; // a contained file's size
FileInputStream fis = new FileInputStream(file );
fis.skip(offset);
parse(fis, size);

public void parse(InputStream is, long size) {
   // parse stream data and insure we don't read more than size bytes
   is.close();
}

I feel like this is no good practice. Is there a better way to do this, maybe using buffering?

Furthermore, I feel like the skip() method slows the reading process a lot.

Answer 1

It sounds like what you really want is a sort of "partial" input stream - one a bit like the ZipInputStream, where you've got a stream within a stream.

You could write this yourself, proxying all InputStream methods to the original input stream making suitable adjustments for offset and checking for reading past the end of the subfile.

Is that the sort of thing you're talking about?

Answer 2

First, FileInputStream.skip() has a bug which may make the file underneath skip beyond the EOF marker of the file so be wary of that one.

I've personally found working with Input/OutputStreams to be a pain compared to using FileReader and FileWriter and you're showing the main issue I have with them: The need to close the streams after using. One of the issues is that you can never be sure if you've closed up all the resources properly unless you make the code a bit too cautious like this:

public void parse(File in, long size) {
    try {
        FileInputStream fis = new FileInputStream(in);
        // do file content handling here
    } finally {
        fis.close();
    }
    // do parsing here
}

This is of course bad in the sense that this would lead to creating new objects all the time which may end up eating a lot of resources. The good side of this is of course that the stream will get closed even if the file handling code throws an exception.