Mapping Map<String, List<>> with List<>

Question

I need some help mapping a Map<String, List<Fee>> with a List<FeeRequest>.

Fee object looks like this:

private String feeCode;

FeeRequest objects looks like this:

private String feeCode;
private String debtorAccount;

So what I need is to map:

String debtorAccount(from map) -> debtorAccount(from List)

feeCode(from List from map) -> feeCode(from List)

I want to try not to use foreach, but instead learn to properly use .stream().map() features.

What I've managed to do:

Map<String, List<Fee>> feeAccounts is parsed from another method.

 List<FeeRequest> feeRequests = feeAccounts.entrySet().stream().map(feeAcc -> {
        FeeRequest request = new FeeRequest();
        request.setDebtorAccount(feeAcc.getKey());
        request.setFeeCode(...);
        return request;
    }).collect(Collectors.toList());

I think that my approach is bad, but I don't know how to make it work. I tried looking at some examples but they're too basic. So I would be glad to get any help. Thanks!

@Eran because I can't map the feeCode, it doesn't show me the Fee objects fields — developer1
– developer1, Commented Jul 18, 2019 at 7:44
you can access the List<Fee> with feeAcc.getValue(). It's not clear what you wish to do with it, though. — Eran
– Eran, Commented Jul 18, 2019 at 7:45
@Eran Basically what I want is to FeeRequest.setFeeCode(Fee.getFeeCode()). Feecodes from one list map with feecodes from another — developer1
– developer1, Commented Jul 18, 2019 at 7:48
But each entry of the input Map contains a List of Fee instances. Which one of them do you wish to use? Or do you want to create multiple FeeRequest instances (one for each Fee instance)? — Eran
– Eran, Commented Jul 18, 2019 at 7:51

Eran · Accepted Answer · 2019-07-18 07:54:37Z

1

If each Fee instance should generate a FeeRequest instance, you need flatMap:

List<FeeRequest> feeRequests =
    feeAccounts.entrySet()
               .stream()
               .flatMap(feeAcc -> feeAcc.getValue()
                                        .stream()
                                        .map(f -> {
                                            FeeRequest request = new FeeRequest();
                                            request.setDebtorAccount(feeAcc.getKey());
                                            request.setFeeCode(f.getCode());
                                            return request;
                                        }))
               .collect(Collectors.toList());

answered Jul 18, 2019 at 7:54

Eran

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1 Comment

arnonuem Over a year ago

Yes because you get a lot of lists in the first step which then have to be merged into one map.

Sasha Shpota · Accepted Answer · 2019-07-18 07:57:47Z

0

If FeeRequest has a constructor with two arguments you can use something like this:

feeAccounts.entrySet().stream()
        .flatMap(
                accountEntry -> accountEntry.getValue().stream().map(
                        fee -> new FeeRequest(
                                accountEntry.getKey(),
                                fee.getFeeCode()
                        )
                )
        ).collect(toList());

answered Jul 18, 2019 at 7:57

Sasha Shpota

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