Java 8 如何使用Java8 Collectors groupingBy获取带有集合映射的映射?
想象一下这些课程Java 8 如何使用Java8 Collectors groupingBy获取带有集合映射的映射?,java-8,java-stream,collectors,Java 8,Java Stream,Collectors,想象一下这些课程 class Subject { private int id; private Type type; private String origin; private String name; Subject(int id, Type type, String origin, String name) { this.id = id; this.type = type; this.origin = origin;
class Subject {
private int id;
private Type type;
private String origin;
private String name;
Subject(int id, Type type, String origin, String name) {
this.id = id;
this.type = type;
this.origin = origin;
this.name = name;
}
// Getters and Setters
}
enum Type {
TYPE1,
TYPE2
}
我有一份这些科目的清单
List<Subject> subjects = Arrays.asList(
new Subject(1, Type.TYPE1, "South", "Oscar"),
new Subject(2, Type.TYPE2, "South", "Robert"),
new Subject(3, Type.TYPE2, "North", "Dan"),
new Subject(4, Type.TYPE2, "South", "Gary"));
List subjects=Arrays.asList(
新主题(1,Type.TYPE1,“南方”、“奥斯卡”),
新主题(2,Type.TYPE2,“南方”、“罗伯特”),
新主题(3,Type.TYPE2,“北”、“丹”),
新科目(4个,类型2,“南方”、“加里”);
我想获得使用Collectors.groupingBy()
映射的结果,首先按Subject.origin
对Subject
对象进行分组,然后按Subject.type
因此,获取这样的对象
Map
groupingBy
会接受一个下游收集器,它也可以是一个groupingBy
:
subjects.stream()
.collect(groupingBy(
Subject::getOrigin,
groupingBy(Subject::getType)
));
Stream subjects=Stream.of(
新主题(1,Subject.Type.TYPE1,“南方”、“奥斯卡”),
新主题(2,Subject.Type.TYPE2,“南方”,“罗伯特”),
新主题(3,Subject.Type.TYPE2,“北”、“丹”),
新科目(4,Subject.Type.TYPE2,“南方”、“加里”);
地图主题ByOriginByType;
subjectByOriginByType=subjects.collect(Collectors.groupingBy(
s->s.getOrigin(),Collectors.groupingBy(
s->s.getType(),
Collectors.mapping((Subject s)->s,Collectors.toList());
这帮助我解决了一个类似的问题:如果您希望进一步聚合叶列表,内部GroupBy还可以使用下游收集器。在我的例子中,我需要.collect(groupingBy(DishIngredientMass::getDishId,groupingBy(DishIngredientMass::getingredientmass,summingDouble(DishIngredientMass::getCookMass))
Stream<Subject> subjects = Stream.of(
new Subject(1, Subject.Type.TYPE1, "South", "Oscar"),
new Subject(2, Subject.Type.TYPE2, "South", "Robert"),
new Subject(3, Subject.Type.TYPE2, "North", "Dan"),
new Subject(4, Subject.Type.TYPE2, "South", "Gary"));
Map<String, Map<Subject.Type, List<Subject>>> subjectByOriginByType;
subjectByOriginByType = subjects.collect(Collectors.groupingBy(
s->s.getOrigin(), Collectors.groupingBy(
s->s.getType(),
Collectors.mapping((Subject s) -> s, Collectors.toList()))));