Convert enum array with set of nested enums to Map

Question

Cannot understand how to compose Map<InternalErrorCode, ExternalErrorCode> variable from the following enum structure using Stream API:

@Getter
@RequiredArgsConstructor
public enum ExternalErrorCode {

    // Internal error codes won't be duplicated across any ExternalErrorCode enums
    ERROR1(Set.of(InternalErrorCode.FOO, InternalErrorCode.BAR)),
    ERROR2(Set.of(InternalErrorCode.ZOO)),
    ...;

    // Expected output should be: [{"FOO","ERROR1"}, {"BAR","ERROR1"}, {"ZOO","ERROR2"}]
    private static final Map<InternalErrorCode, ExternalErrorCode> LOOKUP_BY_ERROR_CODE = Stream.of(ExternalErrorCode.values())
                                                                                           .filter(not(externalErrorCode -> externalErrorCode.getErrorCode().isEmpty()))
                                                                                           .collect(groupingBy(...)); // Here is unclarity

    private final        Set<InternalErrorCode>                            errorCode;

}

Can someone assist with it?

You’re better off with a nested loop.

Holger
– Holger

2021-08-30 15:40:25 +00:00
Commented Aug 30, 2021 at 15:40 — Holger
– Holger, Commented Aug 30, 2021 at 15:40

DuncG · Accepted Answer · 2021-11-14 18:23:12Z

1

The declaration of the lookup LOOKUP_BY_ERROR_CODE can use a Stream with flatMap which expands all combinations of known InternalErrorCode to ExternalErrorCode mappings. There is no need to check for empty sets as these will just be mapped as an empty stream, and finally the collector assembles the entries as a Map:

private static final Map<InternalErrorCode, ExternalErrorCode> LOOKUP_BY_ERROR_CODE
    = Stream.of(ExternalErrorCode.values())
            .flatMap(e -> e.errorCode.stream().map(i -> Map.entry(i, e)))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

=>

LOOKUP_BY_ERROR_CODE={ZOO=ERROR2, BAR=ERROR1, FOO=ERROR1}

The above code will detect if there is duplicate mappings from internal to external - this gets reported by Collectors.toMap as:

java.lang.ExceptionInInitializerError 
Caused by: java.lang.IllegalStateException Duplicate key XXX

edited Nov 14, 2021 at 18:23

answered Sep 7, 2021 at 14:36

DuncG

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