java - 收集Java流中字段的总和

考虑以下类别：

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

    ...

我该如何编写与第一种方法（getRemainingCompanyBalance）相同的Java流“管道”。此代码将导致错误。


  该列表包含每个部门的单独列表。子列表中的每个条目都是该类的一个实例。名称/费用：
  
  
    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，然后从余额中减去费用

最佳答案

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

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

   return initialBalance - sum;

}