java 8 流 groupingBy 到自定义对象的集合

Question

我有以下class结构


public class Store {
    private Long storeId;

    private Long masterStoreId;

    private String operatorIdentifier;
}

public class StoreInfo {

    private String operatorIdentifier;

    private Set<Long> slaveStoreIds;

    public StoreInfo(String operatorIdentifier, Set<Long> slaveStoreIds) {
        super();
        this.operatorIdentifier = operatorIdentifier;
        this.slaveStoreIds = slaveStoreIds;
    }

}

我想将“List”中。是否可以在单个 operation/iteration 中完成？

List<Store> stores;

Map<Long, Set<Long>> slaveStoresAgainstMasterStore = stores.stream().collect(Collectors
                .groupingBy(Store::getMasterStoreId, Collectors.mapping(Store::getStoreId, Collectors.toSet())));

Map<Long, StoreInfo> storeInfoAgainstMasterStore = stores.stream()
                .collect(
                        Collectors
                                .toMap(Store::getMasterStoreId,
                                        val -> new StoreInfo(val.getOperatorIdentifier(),
                                                slaveStoresAgainstMasterStore.get(val.getMasterStoreId())),
                                        (a1, a2) -> a1));

Answer 1

由于 masterStoreId 和 operatorIdentifier 在组中相同（已在评论中确认），您可以通过使用 AbstractMap.SimpleEntry 创建一对来进行分组。然后使用 Collectors.toMap 创建地图。

Map<Long, StoreInfo> storeInfoMap = 
    stores.stream()
          .collect(Collectors.groupingBy(
                      e -> new AbstractMap.SimpleEntry<>(e.getMasterStoreId(),
                                                        e.getOperatorIdentifier()),
                      Collectors.mapping(Store::getStoreId, Collectors.toSet())))
          .entrySet()
          .stream()
          .collect(Collectors.toMap(e -> e.getKey().getKey(),
                            e -> new StoreInfo(e.getKey().getValue(), e.getValue())));

Answer 2

为了完成实施，您正在尝试。您需要确保 StoreInfo 内的合并能力，例如：

public StoreInfo(String operatorIdentifier, Long slaveStoreId) {
    this.operatorIdentifier = operatorIdentifier;
    this.slaveStoreIds = new HashSet<>();
    this.slaveStoreIds.add(slaveStoreId);
}

public static StoreInfo mergeStoreInfo(StoreInfo storeInfo1, StoreInfo storeInfo2) {
    Set<Long> slaveIds = storeInfo1.getSlaveStoreIds();
    slaveIds.addAll(storeInfo2.getSlaveStoreIds());
    return new StoreInfo(storeInfo1.getOperatorIdentifier(), slaveIds);
}

这将简化收集器的实现，您可以相应地调用这些：

Map<Long, StoreInfo> storeInfoAgainstMasterStore = stores.stream()
        .collect(Collectors.toMap(Store::getMasterStoreId,
                store -> new StoreInfo(store.getOperatorIdentifier(), store.getStoreId()),
                StoreInfo::mergeStoreInfo));

java 8 流 groupingBy 到自定义对象的集合

java 8 stream groupingBy into collection of custom object

java

java-8

java-stream

collectors