java - 收集java流中字段的总和

Question

考虑以下类:

public class PersonalExpense {
    private String name;
    private int currentExpenses;

    ...

我如何编写一个java流“管道”，其工作方式与第一个方法(getRemainingCompanyBalance)相同。此代码会导致错误。

the list contains separate lists for each department. each entry in the sub-list is an instance of the class. name/cost:

de#1first"/9900, de#1second"/8700, de#2first"/8500, de#2second"/10000, de#3first"/7800, de#3second"/6900

    private static long getRemainingCompanyBalance ( long initialBalance, List<ArrayList<PersonalExpense>> total) {
        long remainingBalance = initialBalance;
        for (List<PersonalExpense> departmentExpense : total) {
            for (PersonalExpense personalExpense : departmentExpense) {
                System.out.println(personalExpense.getName());
                remainingBalance = remainingBalance - personalExpense.getCurrentExpenses();
            }
        }
        return remainingBalance;
    }

    public static long getRemainingCompanyBalanceLambda ( long initialBalance, List<ArrayList<PersonalExpense>> total) {
        long remainingBalance = initialBalance;


        Integer sum = total
        .stream()
        .flatMap(Collection::stream)
        .filter(pe -> pe instanceof PersonalExpense)
        .map (pe -> (PersonalExpense) pe)
        .collect(Collectors.toList())
        .mapToInt(PersonalExpense::getCurrentExpenses)
        .sum();


        return remainingBalance -= sum;
    }

}

我正在尝试收取费用，然后从余额中减去它们

Answer 1

public static long getRemainingCompanyBalanceLambda ( long initialBalance, List<ArrayList<PersonalExpense>> total) {

   int sum = total.stream()
        .flatMap(List::stream)
        .mapToInt(PersonalExpense::getCurrentExpenses)
        .sum();

   return initialBalance - sum;

}