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copied!<p>It's very difficult to maintain a 1:1 mapping between a sequence of Java chars (which are effectively UTF-16) and bytes which could be anything depending on your file encoding. Even with UTF-8, the "obvious" mapping of 1 byte to 1 char only works for ASCII. Neither UTF-16 nor UTF-8 guarantees that a unicode character can be stored in a single machine <code>char</code> or <code>byte</code>.</p> <p>I would maintain my window into the file as a byte buffer, not a char buffer. Then to find line endings in the byte buffer, I'd encode the Java string <code>"\r\n"</code> (or possibly just <code>"\n"</code>) as a byte sequence using the same encoding as the file is in. I'd then use that byte sequence to search for line endings in the byte buffer. The position of a line ending in the buffer + the offset of the buffer from the start of the file maps exactly to the byte position in the file of the line ending.</p> <p>Appending lines is just a case of seeking to the end of the file and adding your new lines. Changing lines is more tricky. I think I would maintain a list or map of byte positions of changed lines and what the change is. When ready to write the changes:</p> <ol> <li>sort the list of changes by byte position</li> <li>read the original file up to the next change and write it to a temporary file.</li> <li>write the changed line to the temporary file.</li> <li>skip the changed line in the original file.</li> <li>go back to step 2 unless you have reached the end of the original file</li> <li>move the temp file over the original file. </li> </ol>

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