firstClassCollection6.java

public class PayGroups {
  private List<Pay> pays;
  
  public PayGroups(List<Pay> pays) {
    this.pays = pays;
  }
  
  public Long getPayPalSum() return getTotalSum(pay -> PayType.isPayPal(pay.getPayType()));
  
  public Long getVisaSum() return getTatalSum(pay -> PayType.isVisa(pay.getPayType()));
  
  public Long getTotalSum(Predicate<Pay> predicate) {
    return pays.stream()
                .filter(predicate)
                .mapToLong(Pay::getAmount)
                .sum();
  }
}

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)
                  );
  
  PayGroups payGroups = new PayGroups(pays);
  
  Long visaExpenditure = payGroups.getVisaSum();
}