Reading Files - Beginning Java - page 531

How It Works
All you had to do to write the file as well as read it was to create a Path object for the file and create a file
channel that is opened for both reading and writing. You can read and write sequentially or at random.
You read and write the file using essentially the same mechanism. You call the position() method for
the channel object to set the file position and then you call the read() or write() method with buf as
the argument.
You could have used the channel read() and write() methods that explicitly specify the position where
the data is to be read or written as an argument. In this case you would need to cast channel to type
FileChannel . One problem with the example as it stands is that some of the selections could be 99999L ,
which is patently not prime. I got some of these after running the example just three times. You could fix
this by checking each value that you store in the primes array:

for(int i = 0 ; i < PRIMESREQUIRED ; ++i)
{

while(true)
{

index = LONG_BYTES*(int)(PRIMECOUNT*Math.random());
channel.position(index).read(buf); // Read at a random

position
buf.flip(); // Flip the buffer

primes[i] = buf.getLong(); // Extract the prime

if(primes[i] != REPLACEMENT) {
break; // It's good so exit the

inner loop
} else {
buf.clear(); // Clear ready to read

another
}

}
buf.flip(); // Flip to ready for

insertion
buf.putLong(REPLACEMENT); // Replacement into buffer

buf.flip(); // Flip ready to write

channel.position(index).write(buf); // Write the replacement

to file
buf.clear(); // Reset ready for next

read
}

RandomReadWrite.java

This code fragment is commented out in the download. The while loop now continues if the value read
from the file is the same as REPLACEMENT , so another random file position is selected. This continues
until something other than the value of REPLACEMENT is found. Of course, if you run the example often
enough, you won't have enough primes in the file to fill the array, so the program loops indefinitely look-
ing for something other than REPLACEMENT . You could deal with this in several ways. For example, you