地图和 Groupby 一口气

Question

我有一个模型类如下：public class CCP implements Serializable{    private static final long serialVersionUID = 1L;    @Id    @Column(name = "p_id")    private Integer pId;    @Id    @Column(name = "c_id")    private Integer cId;    @Column(name = "priority")    private Integer priority;}我有以下要求：转换List<CCP>成Map<pid, List<cid>>也就是说，我想将 CCP 对象列表转换为以 pid 为键、以关联 cid 列表为值的映射。我尝试了以下事情：Map<Integer, List<CCP>> xxx = ccplist.stream()                .collect(Collectors.groupingBy(ccp -> ccp.getPId()));但这仅给出了 CCP 的列表。我如何在此处获取 cid 列表而不是 CCP？

Answer 1

使用mapping：

Map<Integer, List<Integer>> xxx = 
    ccplist.stream()
           .collect(Collectors.groupingBy(CCP::getPId,
                                          Collectors.mapping(CCP::getCId,
                                                             Collectors.toList())));

Answer 2

ccplist.stream()
         .collect(Collectors.groupingBy(
               CCP::getPId,
               Collectors.mapping(CCP::getCId, Collectors.toList())));