Question

How can I access ‘directly’ the data between 200th and 300th characters in a file in a secondary storage device?

Answer 1

Use a java.io.RandomAccessFile. Pseudocode:

byte[] buffer = new byte[100];
RandomAccessFile r = new RandomAccessFile("path/to/file", "r");
r.seek(200);
r.read(buffer, 0, 100);

Just add error-checking for robustness 😉

Oh, and you said you wanted to read characters, but file operations work on bytes. If you want to read characters then you have to worry about what the file encoding is. If the encoding is something like UTF-8 then you cannot just skip to a fixed byte index, because each UTF character can encode to a variable number of bytes. In this case you will just have to read the file from the start.