Reaching a specific line in a file using RandomAccessFile

Question

Is it possible to position cursor to the start of a specific line in a file through RandomAccessFile?

For e.g. I want to change String starting at char 10 till 20 in line 111 in a file. The file has fixed length records.

Is it possible to directly position the cursor to start of line 111 using RandomAccessFile ?

Update:

I used the following code. However, its returning null.

Line length is 200 characters (which is 200 bytes if I am not wrong)

File f = new File(myFile); 
RandomAccessFile r = new RandomAccessFile(f,"rw"); 
r.skipBytes(200 * 99);   // linesize * (lineNum - 1) 
System.out.println(r.readLine());

Where am I going wrong ?

Answer 1

I'm not sure but seems RandomAccessFile do not support such functionality. As RAF operates with bytes we can skip specific amount of bytes, and if your file has fixed line width this can be achieved by

file.skipBytes(110 * lineSizeInBytes);

Otherwise, you need something like this:

for (int i = 0; i < 110; i++) file.readLine();
String line = file.readLine();

Answer 2

You cannot do this directly with RandomAccessFile. It is attended to work with binary files and helps you to read and write such files fragment at any random location you want. This is why the class is called RandomAccessFile.

But it does no work with texts, so it does not have a way to recognize end of line and does not work in terms of lines at all.

So, to implement what you want you should use BufferedReader, read line-by-line and if you want store position where each line is started, so you will be able to skip required number of bytes to jump to the beginning of needed line.