BalanceAlertsGenerator.java

public static List<KeyValue<SpecificRecord, OutboundMessage>> generateAlerts(AccountEntry accountEntry,
                                                                             CustomerAlertSettings settings) {
      /* Generates addressed alerts for an AccountEntry, using the alert settings with the following steps:
      *  1) Settings are for a specific account, drop AccountEntries not for this account
      *  2) Match each setting with all alerts to generate appropriate messages
      *  3) Address the generated messages
      */

  if (settings == null) {
    return new ArrayList<>();
  }

  return settings.getAccountAlertSettings().stream()
      .filter(accountAlertSettings -> matchAccount(accountEntry, accountAlertSettings))
      .flatMap(accountAlertSettings -> accountAlertSettings.getSettings().stream())
      .flatMap(accountAlertSetting -> Stream.of(
          generateBalanceAbove(accountEntry, accountAlertSetting),
          generateBalanceBelow(accountEntry, accountAlertSetting),
          generateCreditedAbove(accountEntry, accountAlertSetting),
          generateDebitedAbove(accountEntry, accountAlertSetting))
      )
      .filter(Optional::isPresent).map(Optional::get)
      .flatMap(messageWithChannels -> mapAddresses(messageWithChannels.getValue0(), settings.getAddresses())
          .map(address -> KeyValue.pair(address, messageWithChannels.getValue1())))
      .collect(toList());
}

Full code at https://github.com/Axual/rabo-alerts-blog-post/blob/master/src/main/java/BalanceAlertsGenerator.java