import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
public class Main {
public static void main(String[] args) throws Exception {
RandomAccessFile f = new RandomAccessFile("hello.txt", "rw");
FileChannel fc = f.getChannel();
MappedByteBuffer mbb = fc.map(FileChannel.MapMode.READ_WRITE, 0, f.length());
int len = (int) f.length();
for (int i = 0, j = len - 1; i < j; i++, j--) {
byte b = mbb.get(i);
mbb.put(i, mbb.get(j));
mbb.put(j, b);
}
fc.close();
}
}