import java.io.File;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
public class Main {
public static void main(String[] argv) throws Exception {
File file = new File("filename");
FileChannel channel = new RandomAccessFile(file, "rw").getChannel();
MappedByteBuffer buf = channel.map(FileChannel.MapMode.READ_WRITE, 0, (int) channel.size());
buf.put(0, (byte) 0xFF);
buf.force();
channel.close();
}
}