 |
 | Home | Contact Us | FAQ | Search & Site Map | Link to Us |
 | Sign In | Join | Other 45 Sites in Network |
Java Forum / General / March 2008In need of something like RandomAccessFile.read(char[] cAr, int off, int len)
~ http://java.sun.com/j2se/1.4.2/docs/api/java/io/RandomAccessFile.html ~ has: ~ public int read(byte[] b, int off, int len) throws IOException ~ But I need to read in and compare chars/Unicode ~ I have tried many things but I haven't been able to find out how ~ What kind of carpentry do you do with I/O objects to achieve such a thing? ~ Thanks lbrtchx > ~ > http://java.sun.com/j2se/1.4.2/docs/api/java/io/RandomAccessFile.html [quoted text clipped - 15 lines] > Thanks > lbrtchx Besides RandomAccessFile.readChar() you could get a FileChannel, read in a buffer and convert it to a CharBuffer. But seek() and readChar() ought to be adequate.  SignatureKnute Johnson email s/nospam/linux/ ------->>>>>>http://www.NewsDem >> ~ >> [quoted text clipped - 23 lines] > readChar() > ought to be adequate. ISTM that there should be an InputStream subclass that reads bytes from a RandomAccessFile starting at a given offset. It's not difficult to construct, but why doesn't it come standard? >>> ~ >>> [quoted text clipped - 26 lines] > from a RandomAccessFile starting at a given offset. It's not > difficult to construct, but why doesn't it come standard? It does. He wanted to read chars and that is alittle more complicated because you have to read two bytes at a time. Of course that is what he really wanted. SignatureKnute Johnson email s/nospam/linux/ >>>> ~ >>>> [quoted text clipped - 31 lines] > complicated > because you have to read two bytes at a time. If there were an InputStream, you coiuld attach an InputStreamReader to it and read characters in any encoding you like. Unfortunately, there isn't one. > ... are these files written with a Java program? ~ No necessarily. They are mostly texts downloaded from the Internet ~ > ... If there were an InputStream ~ option as an argument to a RandomAccessFile ctor then things were easier, but I think, and I may be wrong, that there is a fundamental problem here. ~ InputStreams and RandomAccessFiles should not be mixed because when you go: ~ RandomAccessFiles.seek((long) lThere) ~ you can not be absolutely sure that: ~ 1) you will land at the start of a byte sequence conforming a character, ~ 2) belonging to the encoding you specified in the InputStream ~ 3) at, ... where actually? lThere? the API says: ~ http://java.sun.com/j2se/1.5.0/docs/api/java/io/RandomAccessFile.html#seek(long) ~ <API> seek: public void seek(long pos) throws IOException ~ Sets the file-pointer offset, measured from the beginning of this file, at which the next read or write occurs. The offset may be set beyond the end of the file. Setting the offset beyond the end of the file does not change the file length. The file length will change only by writing after the offset has been set beyond the end of the file. Parameters: pos - the offset position, measured in bytes from the beginning of the file, at which to set the file pointer. Throws: IOException - if pos is less than 0 or if an I/O error occurs. </API> ~ The unclear bit is "measured from the beginning of this file" how exactly is it measured? Why not simply saying in "bytes"? ~ I think that java should let the programmer do something like what I illustrate with a piece of pseudo code below ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ // __ int iChrsRd, iChrBfrSz = 4096; char[] cChrAr = new char[iChrBfrSz]; RandomAccessFile RAxFl; String aEnc = ...; // "UTF8", "ISO-8859-1", "" or whatever encoding your text are written in // __ try{ FileInputStream FIS = new FileInputStream(IFl); InputStreamReader ISRdr = new InputStreamReader(FIS, aEnc); RAxFl = new RandomAccessFile(ISRdr); // . . . RAxFl.seek(lThere); iChrsRd = RAxFl.read(cChrAr, 0, iChrBfrSz); // reading iChrsRd into cChrAr provided iChrBfrSz can fully take them // . . . RAxFl.close(); }catch(FileNotFoundException FlNtFX){ FlNtFX.printStackTrace(); } catch(IOException IOX){ IOX.printStackTrace(); } // __ finally{ if(RAxFl != null){ try{ RAxFl.close(); }catch(IOException IOXcptn) { ; }} } ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Dealing with Java's I/O and internationalization is not exactly easy ~ lbrtchx > What kind of carpentry do you do with I/O objects to achieve such a >thing? >~ see http://mindprod.com/jgloss/nio.html You can look at your file as if it were a giant array of char -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com 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 > OK, > ~ [quoted text clipped - 46 lines] > 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. SignatureKnute Johnson email s/nospam/linux/ Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
|
Reply to this Message
|
 Sign In
 Join
 My Latest Posts My Monitored Threads My Blog My Photo Gallery My Profile My Homepage Start New Thread Enable EMail Alerts Rate this Thread
|
©2008 Advenet LLC Privacy Policy - Terms of Use
This website includes both content owned or controlled by Advenet as well as content owned or controlled by third parties.