Java 8? Stream? Groupingby Collector? ?? ??? ??? ?????. pield ??? ?? : ?? ??, City by City, Collectors.groupingby? ????? (person :: getcity); ② ?? ?? ??? : ?? ??, ???? ?? ????? ??? ?? ?? ? ???? ??????. ③ ?? ??? ?? ??? ?? : ?? ??, Collectors.counting ()? ???? ??? ???? Collectors.averagingInt ()? ???? ??? ?????. ④ ??? ? ???? ??????. ?? ??, ??? ???? ??? ?? Collectors.mapping ()? ???? Collectors.joining ()? ??????.

Java 8??? Collectors.groupingBy() ? ?? ? Stream 's collect() ???? ?? ???? ?? ??? ? ? ????. SQL?? GROUP BY ????? ??? ??? ??? ???? ? ? ???? ?????.

?? ?? : ?? ? ??

???? ?? ?? ??? ?? ???? ????? ????. ?? ??, name ? city ????? Person ??? ????. ??? ???? ???? ????? ???? :

 Map <String, list <person >> peopleCity = people.stream ()
    .collect (Collectors.groupingby (person :: getcity));

??? Map ???? ??? ?? ???? ??? ??? Person ?????.

?? : ??? ? ??? Null?? ??? ?????. ??? ?? ?? ??? ????? ??? ?? ???? ???? ???????.

??? ??? : ?? ???

??? ??? ??? ? ??, ??? ?? ?? ???? ?????? ????.

 map <string, map <integer, list <person>>> grouped = people.stream ()
    .collect (Collectors.groupingby (person :: getcity,
             Collectors.groupingby (person :: getage));

??? ??? ??? "?? ?? ???"???? ??? ??? ?? ?? ??? ?? ?? ? ??? ?? Map ???.

??? ?? ?? ??? ?? : ??? ????

????? groupingBy ? ??? ?? ??? ?????. ??? ?? ? ??? ??? ??? ???? ?? ??, ?, ?? ?? ?? ?? ??? ?????.

 // ? ?????? ??? ?? ?? <String, long> countbycity = people.stream ()
    .collect (Collectors.groupingby (person :: getcity, collectors.counting ()));

// ? ?? ?? ?? ?? <String, double> avgageBycity = people.stream ()
    .collect (Collectors.groupingby (person :: getcity,
             Collectors.averagingInt (person :: getage));

???? ?? ??? ???? ??? ????.

• counting() : ?? ??
• summingInt() / summingDouble() : sum
• averagingInt() : ??? ?????
• mapping() : ?? ??? ?? ? ?? ??????

??? ? ??? ?? : TOLIST/TOSET/JOINING ?? ??????.

??? ? ???? ??? ?? ?? ???? ???? Collectors.mapping() ?? Collectors.joining() ??? ? ????.

 // ? ??? ??? ??? ? <???, ???> namesbycity = people.stream ()? ? ??? ????.
    .collect (Collectors.groupingby (person :: getcity,
             Collectors.mapping (Person :: GetName, Collectors.joining ( ",")));

?? ??? ??? ?????.

 {
  ??? = [Zhang San, Li Si],
  ??? = [? ?]
}

????? ??? ???? ??. Java ???? groupingBy ?? ? ???? ?? ???? ??? ??? ?? ?????. ??? "?? ??? ???"? ??? ???? ????. ?? ????? ??? ??? ??? ??? ???? ?? ? ? ????.

? ??? GroupingBy? ?? Java Stream Collect ()? ???? ???? ?? ?????. ??? ??? PHP ??? ????? ?? ?? ??? ?????!