Reading Files - Beginning Java - page 529

You could also have used the read() method that accepts a single argument together with the posi-
tion() method:
for(int i = 0 ; i<PRIMESREQUIRED ; ++i) {
index = LONG_BYTES*(int)(PRIMECOUNT*Math.random());
inCh.position(index).read(buf);

// Read the value

buf.flip();
primes[i] = buf.getLong(); // Save it in the

array

buf.clear();

}

Calling the position() method sets the file position to index then the read() method reads the file
starting at that point.
The need to be able to access and update a file randomly arises quite often. Even with a simple personnel
file, for example, you are likely to need the capability to update the address or the phone number for an
individual. Assuming you have arranged for the address and phone number entries to be of a fixed length,
you could update the data for any entry simply by overwriting it. If you want to read from and write to
the same file you can just open the channel for reading and writing. Let's try that, too.

TRY IT OUT: Reading and Writing a File Randomly
You can modify the previous example so that you overwrite each random prime that you retrieve from
the primes_backup.bin file that you created earlier with the value 99999L to make it stand out from the
rest. This messes up the primes_backup.bin file that you use here, but you can always run the program
that copies files to copy primes.bin if you want to restore it. Here's the code:

import static java.nio.file.StandardOpenOption.*;
import java.nio.file.*;
import java.nio.channels.SeekableByteChannel;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.EnumSet;

public class RandomReadWrite {
public static void main(String[] args)
{

Path file = Paths.get(System.getProperty("user.home")).
resolve("Beginning Java
Stuff").resolve("primes_backup.bin");
if(!Files.exists(file)) {
System.out.println(file + " does not exist. Terminating
program.");
System.exit(1);

}