按多个字段分组的流:，分析映射的嵌套映射的输出[重复]

Question

@Getter
@Setter
@AllArgsConstructor
class SampleAction {
    private String grouping;
    private String category;
    private String actionType;
    private String details;
    private String moreDetails;
    private String etc;
}

java prettyprint-override">List<SampleAction> items = new ArrayList<SampleAction>() {{
    add(new SampleAction("Group1", "Cat1", "Type1", "details1", "", "etc"));
    add(new SampleAction("Group2", "Cat1", "Type1", "details2", "", "etc"));
    add(new SampleAction("Group1", "Cat1", "Type2", "details3", "", "etc"));
    add(new SampleAction("Group1", "Cat2", "Type1", "details4", "", "etc"));
    add(new SampleAction("Group2", "Cat2", "Type2", "details5", "", "etc"));
    add(new SampleAction("Group2", "Cat2", "Type2", "details6", "", "etc"));
    add(new SampleAction("Group2", "Cat3", "Type1", "details7", "", "etc"));
}};

items.stream()
   .collect(Collectors.groupingBy(SampleAction::getGrouping,
            Collectors.groupingBy(SampleAction::getCategory,
            Collectors.groupingBy(SampleAction::getActionType))));

这看起来有点吓人。最好的办法是什么？输出真的需要这种复杂类型吗？

Answer 1

如果您不想处理这种嵌套的映射，可以创建一个组合键来分组：

Map<String,List<SampleAction>> map =
    items.stream()
          .collect(Collectors.groupingBy(sa -> sa.getGrouping () + '/' + 
                                               sa.getCategory() + '/' + 
                                               sa.getActionType()));

当然，只要您想在映射中搜索值，就必须创建一个复合键。

例如：

List<SampleAction> list = map.get (aGroup + '/' + aCategory + '/' + anActionType);