Article: Law of Demeter - company using internal methods

Company.java

class Company {

    private List<Department> departments;

    Map<DepartmentCode, BigDecimal> costPerDepartment() {
        Map<DepartmentCode, BigDecimal> costPerDepartment = new HashMap<>();
        departments.forEach(department -> {
            DepartmentCode departmentCode = department.getCode();
            BigDecimal employeesCost = calculateCostOf(department);
            costPerDepartment.put(departmentCode, employeesCost);
        });
        return costPerDepartment;
    }

    private BigDecimal calculateCostOf(Department department) {
        return department.getTeams().stream()
                         .map(team -> calculateCostOf(team))
                         .reduce(BigDecimal.ZERO, BigDecimal::add);
    }

    private BigDecimal calculateCostOf(Team team) {
        return team.getMembers().stream()
                   .map(Member::getCost)
                   .reduce(BigDecimal.ZERO, BigDecimal::add);
    }
}