@Override
  public byte[] compute(byte[] input, int outputLength) throws GeneralSecurityException {
    if (input == null) {
      throw new GeneralSecurityException("Invalid input provided.");
    }
    if (outputLength <= 0) {
      throw new GeneralSecurityException("Invalid outputLength specified.");
    }
    InputStream prfStream = prfStreamer.computePrf(input);
    return readBytesFromStream(prfStream, outputLength);
  }
Reads the specified number of bytes from the string or throws an exception.
java_src/src/main/java/com/google/crypto/tink/subtle/prf/PrfImpl.java