java 8 alternative for nested for loops

Question

I have the following snippet and I wonder if and how it is possible to replace it with Java-streams/Java 8 API

List<Borrower> borrowers = creditcomplex.getBorrowers();
for (Borrower borrower : borrowers) {
    List<Facility> facilities = borrower.getFaciliies();
    for (Facility facility : facilities) {
        List<RepaymentSchedule> repaymentScheduleList = facility.getrepaymentSchedule();
        if (repaymentScheduleList != null) {
            for (RepaymentSchedule repaymentschedule : repaymentScheduleList) {
                double[] repayment = 
                    amortizationService.calculateAmortizationSchedule(repaymentschedule);
                double[] drawdown = 
                    amortizationService.calculateRepaymentSchedule(repaymentschedule);
                double[] outstandingProfie = amortizationService
                        .calculateOutstandingSchedule(repaymentschedule);
            }
        }
    }
}

I'd probably look to fold some of the above functionality into (say) the Borrower class, the Facility class etc. Then you can ask those classes and delegate, rather than pulling objects out of objects out of objects.... See https://en.wikipedia.org/wiki/Law_of_Demeter for more on this.... You *may* need to pass the amortizationService around, note... — Brian Agnew, Feb 06 '20 at 16:07

Andronicus · Answer 1 · 2020-02-06T16:22:07.207

4

You can use flatMap:

creditcomplex.getBorrowers().stream()
    .flatMap(b -> b.getFaciliies().stream())
    .flatMap(f -> Optional.ofNullable(f.getrepaymentSchedule()).stream())
    .forEach(repaymentschedule -> {
            double[] repayment = 
                amortizationService.calculateAmortizationSchedule(repaymentschedule);
            double[] drawdown = 
                amortizationService.calculateRepaymentSchedule(repaymentschedule);
            double[] outstandingProfie = amortizationService
                    .calculateOutstandingSchedule(repaymentschedule);
    });

P.S.1: Note that Optional#stream appeared in java 9, you might need to use:

optional.map(Stream::of).orElseGet(Stream::empty)

It's taken from here.

P.S.2: What you're doing in a forEach does not have any effect (you're declaring and initializing arrays inside, but you cannot use them outside of the loop. I left the code because it can be replaced with any computation on the nested elements.

P.S.3: Returning null instead of empty list is error prone and it's usually better to go with an empty list.

edited Feb 06 '20 at 16:22

answered Feb 06 '20 at 16:04

Andronicus

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why use flatMap(f -> Optional.ofNullable(f.getrepaymentSchedule()).stream()) instead of juste : map(Facility::getrepaymentSchedule) ??? – Arnault Le Prévost-Corvellec Feb 06 '20 at 16:10
@ArnaultLePrévost-Corvellec because of the `null`-check. In the question it checks: `if (repaymentScheduleList != null)`, so I cannot simply write `f.getrepaymentSchedule().stream()` since that can result in NPE – Andronicus Feb 06 '20 at 16:12
1

@ArnaultLePrévost-Corvellec returning empty list instead of `null` is a better style, I have added it in P.S.3 – Andronicus Feb 06 '20 at 16:14
nop you filter null object in next step of your stream ^^ – Arnault Le Prévost-Corvellec Feb 06 '20 at 16:14
perfectly aggre on list but stream can return null object filters method had been add in this purpose on J8 =) – Arnault Le Prévost-Corvellec Feb 06 '20 at 16:16
@ArnaultLePrévost-Corvellec hah, you're right. I missed that. Thank you very much! – Andronicus Feb 06 '20 at 16:22

score 1 · Answer 2 · answered Feb 06 '20 at 16:10

Untested, but it should roughly be

List<RepaymentSchedule> repaymentSchedules = creditcomplex.getBorrowers().stream()
    .flatMap(borrower -> borrower.getFacilities().stream())
    .map(facility -> facility.getrepaymentSchedule())
    .filter(repaymentScheduleList -> repaymentScheduleList != null)
    .flatMap(repaymentScheduleList -> repaymentScheduleList.stream())
    .collect(Collectors.toList());

would be one thing, and from here youcan create the arrays.

Alternatively, you can omit the .collect() and instead do

.forEach(repaymentSchedule -> {
    double[] repayment = 
                amortizationService.calculateAmortizationSchedule(repaymentschedule);
    double[] drawdown = 
                amortizationService.calculateRepaymentSchedule(repaymentschedule);
    double[] outstandingProfie = amortizationService
                    .calculateOutstandingSchedule(repaymentschedule);
});

java 8 alternative for nested for loops

2 Answers2