Re: Using mark and reset on BufferReader, but with unknown file length

Eric Sosman <esosman@ieee-dot-org.invalid>
Sat, 23 Apr 2011 08:52:16 -0400
On 4/23/2011 7:24 AM, Merciadri Luca wrote:

For some reasons that come out of the scope of this discussion, I need
to read a file twice. I would prefer not to do

BufferedReader inputFile = new BufferedReader(new

twice, and, as a result, I would like to use my preceding
BufferedReader to read the file again, a second time.

That is, I first read the file, then close it, then need to re-read
it. To re-read it, I would like to use reset and mark methods, defined
over BufferedReader objects. The problem is that mark obliges me to
give a number of characters:

readAheadLimit - Limit on the number of characters that may be read while still preserving the mark. After reading this many characters, attempting to reset the stream may fail. A limit value larger than the size of the input buffer will cause a new buffer to be allocated whose size is no smaller than limit. Therefore large values should be used with care.
Unfortunately, I don't know how much characters there will be in my
file, and need to get to the beginning of it for reading it the second
time. Could I use reset() without mark? And, as I can't type a big
integer as readAheadLimit (because it would allocate too much memory), how can I do?

     You cannot use reset() without a prior mark(). (Well, you *can*,
but it's just a fancy way to throw an IOException.) What reset() does
is "rewind" to the mark() position; if there has been no mark() there
is no position to rewind to.

     If your input is re-readable (e.g. a file that's not being changed
by some other activity), opening a fresh Reader is a straightforward
and simple way to re-read it. Alternatively, you could try mark() with
whatever size you're comfortable with, hoping and trusting that the
BufferedReader will exploit the source's re-readability.

     If the input is not re-readable (keyboard, socket, microphone, ...),
then the only way to make a second pass over the data is to store it
on the first pass -- in short, a full-size buffer or other repository
is mandatory.

Eric Sosman

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