import java.io.File;
import java.io.RandomAccessFile;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.charset.Charset;
import java.nio.charset.CharsetDecoder;
import java.nio.charset.CharsetEncoder;
public class Main {
static public void main(String args[]) throws Exception {
File infile = new File("inFilename");
File outfile = new File("outFilename");
RandomAccessFile inraf = new RandomAccessFile(infile, "r");
RandomAccessFile outraf = new RandomAccessFile(outfile, "rw");
FileChannel finc = inraf.getChannel();
FileChannel foutc = outraf.getChannel();
MappedByteBuffer inmbb = finc.map(FileChannel.MapMode.READ_ONLY, 0, (int) infile.length());
Charset inCharset = Charset.forName("UTF8");
Charset outCharset = Charset.forName("UTF16");
CharsetDecoder inDecoder = inCharset.newDecoder();
CharsetEncoder outEncoder = outCharset.newEncoder();
CharBuffer cb = inDecoder.decode(inmbb);
ByteBuffer outbb = outEncoder.encode(cb);
foutc.write(outbb);
inraf.close();
outraf.close();
}
}