Read text file from position in Java

Question

I need to read char[] (size is COUNT) from text file from OFFSET with specified Charset. COUNT and OFFSET are in characters, not in bytes. He is my code:

raf = new RandomAccessFile(filePath, "r");      
if ((mBuffer == null) || (mBuffer.length < count)) {
    mBuffer = new byte[(int)(count/mDecoder.averageCharsPerByte())];
    mByteWrap = ByteBuffer.wrap(mBuffer);
    mCharBuffer = new char[count];
    mCharWrap = CharBuffer.wrap(mCharBuffer);
}
try {
    offset = (int)(offset/mDecoder.averageCharsPerByte());
    count = (int)(count/mDecoder.averageCharsPerByte());
    raf.seek(offset);
    raf.read(mBuffer,0,count);
    mByteWrap.position(0);
    mCharWrap.position(0);
    mDecoder.decode(mByteWrap, mCharWrap, true);
} catch (IOException e) {
    return null;
}
return mCharBuffer;

Is there any way easier ? (without manual matching char->byte)

I was looking about java.util.Scanner, but it's Iterator-style, and i need random access-style.

PS data should'n be copied many times

Solution

Use BufferedReader's skip() method. In your case:

BufferedReader reader = new BufferedReader(new FileReader(filePath));
reader.skip(n); // chars to skip
// .. and here you can start reading

And if you want specify a particular encoding you can use

InputStream is = new FileInputStream(filePath);
BufferedReader reader = new BufferedReader(new InputStreamReader(is,"UTF-8"));
reader.skip(n); // chars to skip
// .. and here you can start reading

Comments

[styanton]

I suppose BufferedReader uses default system charset while reading ? How does skip() works (is it just moves file pointer or reads n chars) ?

[dash1e]

I add to the answer the way you can specify charset.

[styanton]

And if i first need 100-200 characters, then 50-100 ?

[dash1e]

With charset encoding where character length is not fixed is complex. Because you have to start from the beginning or from points where you can be sure that new char start. However you can try using mark() and reset(), and mark immediately the position 0.

[styanton]

I was talking about positioning reader. I've found void reset(). Thanks, dash1e!

[styanton]

Looks like BufferedReader().read(char buf[]) usede additional memory, except target buffer. Calling it in cycle makes heap size grow and dalvik (i'm runing under Android) cause GC_FOR_ALLOC. Later OutOfMemory error is thrown. P.S. before every read i call reader.reser(), that is set to the file beginingg, using reader.mark(Integer.MAX_VALUE). I think i should mark by another value.