private String computeHkdfHex(String macAlgorithm, String ikmHex, String saltHex, String infoHex,
      int size) throws GeneralSecurityException {
    return Hex.encode(
        Hkdf.computeHkdf(
            macAlgorithm, Hex.decode(ikmHex), Hex.decode(saltHex), Hex.decode(infoHex), size));
  }
Test version of Hkdf where all inputs and outputs are hexadecimal.
java_src/src/test/java/com/google/crypto/tink/subtle/HkdfTest.java