Re: In need of something like RandomAccessFile.read(char[] cAr, int off, int len)

From:

Knute Johnson <nospam@rabbitbrush.frazmtn.com>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 12 Mar 2008 15:32:51 -0700

Message-ID:

<47d82297$0$1650$b9f67a60@news.newsdemon.com>

lbrtchx@gmail.com wrote:

OK,
~
I am trying to do something like this.
~
. . .
   FileInputStream FIS = new FileInputStream(new File(aIFl));
   FileChannel FlChnl = FIS.getChannel();
   long lFlChnlL = FlChnl.size();
   MappedByteBuffer MptBytBfr =
FlChnl.map(FileChannel.MapMode.READ_ONLY, 0, lFlChnlL);
// __
   String aChrSet = "ISO-8859-1"; // UTF8 or whatever
   Charset ChrSt = Charset.forName(aChrSet);
   CharsetDecoder ChrStDkdr = ChrSt.newDecoder();
   CharBuffer ChrBfr = ChrStDkdr.decode(MptBytBfr);
   char[] cArFl = ChrBfr.array();
. . .
~
and I know the offsets in the files where certain sequences appear
and the length of the sequences, so then I go:
~
. . .
   for(int i = 0; (i < iSeqL); ++i){ aB.append(cArFl[((int)lFfst +
i)]); }
   aS = aB.toString();
. . .
~
to grab the actual sequence. However it does not seem to be working.
Can you, please, point me to a full example out there?
~
I like that you can set a CharsetDecoder to the actual file since I
may be using non "ISO-8859-1" text files, but I still wonder about:
~
1. what the speed gains really are
~
2. if "offsets" somehow change based on the CharsetDecoder
~
3. how safe the use of Memomy maps + CharsetDecoder while reading
file-based data feeds
~
I find java's I/O a bit confusing and I also think that the
RandomAccessFile class should take care of the inner plumbing needed
to offer something like what I need, namely
RandomAccessFile.read(char[] buffer, int start, int end)
~
When I don't see it, I think there may be some rather unsafe issues
underlying that reason.
~
Thanks
lbtchx

So are these files written with a Java program and do they use the Java
16 bit unicode characters? Do you actually need random access or are
you just going to read parts out of the file once and go on? If the
characters are encoded in full unicode how would you know where to look
for them? If the files are encoded in ASCII or UTF-8 you should be able
to use a BufferedReader and select an appropriate character set. Just
skip over the bytes you don't want to read.

--

Knute Johnson
email s/nospam/linux/

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