firstClassCollection5.java

public void managingStateAndAction() {
  List<Pay> pays = Arrays.asList(
                    new Pay(PAYPAL, 1000),
                    new Pay(VISA, 2000),
                    new Pay(MASTER_CARD, 3000),
                    new Pay(UNION_CARD, 5000)
                  );
  
  Long totalSum = pays.stream().filter(pay -> pay.getPayType().equals(VISA)).mapToLong(Pay::getAmount).sum();
}