详解Java8的groupBy实现集合的分组

Map<String, List<EntryDeliveryDetailywk>> detailsMap01 = dtos1.stream()
.collect(Collectors.groupingBy(EntryDeliveryDetailywk::getskuId));

Map<String, List<EntryDeliveryDetailywk>> detailmap = details.stream()
.collect(Collectors.groupingBy(d -> fetchGroupKey(d) ));

private String fetchGroupKey(EntryDeliveryDetailywk detail){
  return detail.getSkuId().toString() 
  + detail.getItemsName() 
  + detail.getWarehouseId().toString() 
  + detail.getSupplierId().toString();
 }

//静态内部类
class Person {
 public static class NameAge {
  public NameAge(String name, int age) {
   ...
  }

  // 注意 重写方法 must implement equals and hash function
 }

 public NameAge getNameAge() {
  return new NameAge(name, age);
 }
}

//分组
Map<NameAge, List<Person>> map = people.collect(Collectors.groupingBy(Person::getNameAge));

Map<String, Map<Integer, List<Person>>> map = people
 .collect(Collectors.groupingBy(Person::getName,
  Collectors.groupingBy(Person::getAge));

//调用
map.get("Fred").get(18);