java - 将值复制到 Map<DemoType, List<Demography>>

Question

我对这段代码有一些问题。

我正在创建一个包含人口统计列表的 map ，但在某些时候我有重复的列表元素。你能看出我哪里做错了吗？

问候。

public Map<DemoType, List<Demography>> demoTypeToDemosList(final Long campaignId, final MediaType mediaType) {
        List<MappedDemoDTO> mappedDemosList = demoTypeCampaignService.getMappedDemosByMediaTypeAndCampaignIdOrderByOrdinalRankAsc(mediaType, campaignId);

        final Set<Long> demoIds = mappedDemosList.stream().map(MappedDemoDTO::getMappedId).collect(Collectors.toSet());
        DemographyProviderService serviceByProviderAndMediaType = demographyFactory.getServiceByProviderAndMediaType(Provider.PROVIDER, mediaType);
        final List<Demo> demos = serviceByProviderAndMediaType.getDemoByExternalIds(demoIds);

        Map<DemoType, List<Demography>> demoTypeToDemosMap = new EnumMap<>(DemoType.class);

        mappedDemosList.forEach(mappedDemo -> demos.stream()
            .filter(demo -> mappedDemo.getMappedId().equals(demo.getId()))
            .findFirst()
            .ifPresent(demo -> {
                List<Demography> demosList = demoTypeToDemosMap.computeIfAbsent(mappedDemo.getDemoType(), f -> new ArrayList<>());
                demo.setId(mappedDemo.getDemoId());
                demosList.add(demo);
            }));

        return demoTypeToDemosMap;
    }

Answer 1

阅读computeIfAbsent的文档:

return the current (existing or computed) value associated with the specified key, or null if the computed value is null

这个:

List<Demography> demosList = demoTypeToDemosMap.computeIfAbsent(mappedDemo.getDemoType(), f -> new ArrayList<>());

如果键不在 Map 或现有的键中，

将返回一个新的 ArrayList，您稍后只需将其添加到其中即可: demosList.add(demo);