ProjectBillingService.java

class ProjectBillingService {
    private final CustomerContract contract;
    private final DeveloperHistoryRepository repository;
    /*...*/
    public ProjectBillingService(DeveloperHistoryRepository repository, CustomerContract contract) {
        this.repository = repository;
        this.contract = contract;
    }
    /*...*/
    public BigDecimal calculateCost(YearMonth yearMonth){
        Date startDate = getStartDate(yearMonth);
        Date endDate = getEndDate(yearMonth);
        List<DeveloperHistory> history = repository.findBy(contract.id(), startDate, endDate);
        return contract.applyFeeTo(developersCost(history, contract, yearMonth));
    }

    private BigDecimal developersCost(List<DeveloperHistory> history,
                                      CustomerContract contract,
                                      YearMonth yearMonth) {
        return history.stream()
                .map(developerHistory -> costPerDeveloper(developerHistory,
                        contract, yearMonth))
                .reduce(new BigDecimal(0), BigDecimal::add);
    }

    private BigDecimal costPerDeveloper(DeveloperHistory developerHistory,
                                        CustomerContract contract,
                                        YearMonth yearMonth){
        return multiply(
                contract.dailyRateFor(
                        developerHistory.seniorityLevelOn(yearMonth)),
                developerHistory.billableDaysOn(yearMonth, contract.id()));
    }

    private BigDecimal multiply(BigDecimal dailyRate, float numberOfWorkingDays) {
        return dailyRate.multiply(BigDecimal.valueOf(numberOfWorkingDays));
    }

    private Date getEndDate(YearMonth yearMonth) {
        return new Date(); // TODO: implement this
    }

    private Date getStartDate(YearMonth yearMonth) {
        return new Date(); // TODO: implement this
    }
}