I think your code is quite ok, but you are still working with String to create the byte value, so you maybe better change String.valueOf(in[i]);
to something like this:
public static String digest(char[] in) throws NoSuchAlgorithmException {
MessageDigest md = MessageDigest.getInstance("SHA-256");
ArrayList<Byte> list = new ArrayList<Byte>();
for(int i = 0; i<in.length; i++){
byte b = (byte) in[i]
list.add(b);
}
byte[] inputInByte = new byte[list.size()];
for(int i =0;i<list.size();i++){
inputInByte[i] = list.get(i);
}
md.update(inputInByte);
byte byteData[] = md.digest();
StringBuffer hexString = new StringBuffer();
for (int i = 0; i < byteData.length; i++) {
String hex = Integer.toHexString(0xff & byteData[i]);
if (hex.length() == 1) {
hexString.append('0');
}
hexString.append(hex);
}
return hexString.toString();
}
that is also easier than using that for cycle and two step conversion to byte.