@Override
  public byte[] compute(byte[] data, int outputLength) throws GeneralSecurityException {
    if (outputLength > maxOutputLength) {
      throw new InvalidAlgorithmParameterException("tag size too big");
    }

    localMac.get().update(data);
    return Arrays.copyOf(localMac.get().doFinal(), outputLength);
  }
Given an HmacPrfKey, returns an instance of the Prf interface.
java_src/src/main/java/com/google/crypto/tink/subtle/PrfHmacJce.java