[자바 8] 람다 스트림 예문 PersonService.java (출처: 자바의정석)

package com.wego.web.person;
import java.util.List;
import java.util.Map;
import java.util.Optional;
import java.util.Set;
import java.util.Comparator;
import java.util.IntSummaryStatistics;
import java.util.stream.Collectors;
import  org.springframework.beans.factory.annotation.Autowired;
import  org.springframework.stereotype.Service;
@Service
public class PersonService {
    @Autowired private PersonRepository  personRepository;
    
    // 자바의 정석 847 collect()
    public List<String> namesOfStudents() {
       // 1. 학생의 이름
       return  personRepository.findByRole("student").stream()
               .map(Person::getName)
               .collect(Collectors.toList());
    }
    public Person[] streamToArray() {
       // 2. 스트림을 배열로 전환
       return  personRepository.findByRole("student").stream()
               .toArray(Person[]::new);
    }
    public Map<String, Person> streamToMap()  {
       // 3. 스트림을 맵으로 전환. 학생이름이  key
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.toMap(s->s.getName(),  p->p));
    }
    public Long theNumberOfStudents() {
       // 4. 학생의 수
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.counting());
    }
    public Integer totalScore() {
       // 5. 학생들 총점
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.summingInt(Person::getScore));
    }
    public Optional<Person> topStudent(){
       // 6. 1등 학생
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.maxBy(Comparator.comparingInt(Person::getScore)));
    }
    public IntSummaryStatistics getStat() {
       // 7. 전체 학생 성적 통계
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.summarizingInt(Person::getScore));
    }
    public String nameList() {
       // 8. 전체 학생 이름
       return  personRepository.findByRole("student").stream()
               .map(Person::getName)
               .collect(Collectors.joining(",","{","}"));
    }
    
    
    // 자바의정석 851 partioningBy()
    public List<Person>  partioningByGender(boolean gender) {
       // 1.단순분할 (성별로 분할) 남성 true  여성 false
       return  personRepository.findByRole("student").stream()
               .collect(
                      Collectors
                          .partitioningBy(Person::isMale))
               .get(gender);
    }
    public Long  partioningCountPerGender(boolean gender) {
       // 2.단순분할 (성별 학생수)
       return  personRepository.findByRole("student").stream()
               .collect(
                      Collectors
                          .partitioningBy(Person::isMale,
                              Collectors.counting()))
               .get(gender);
    }
    public Person  partioningTopPerGender(boolean gender) {
       // 3.단순분할 (성별 1등) 남성 true  여성 false
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.partitioningBy(
                      Person::isMale,
                      Collectors.collectingAndThen(
                              Collectors.maxBy(
                                     Comparator.comparingInt(
                                             Person::getScore)),
                                             Optional::get)))
               .get(gender);
    }
    public List<Person>  partioningRejectPerGender(boolean gender) {
       // 4.다중분할 (성별 불합격자,  50점이하) 남성 true 여성 false
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.partitioningBy(Person::isMale,
                      Collectors.partitioningBy(s->s.getScore()<=50)))
               .get(gender)
               .get(true);
    }
    // 자바의정석 855
    public Map<Integer, List<Person>>  groupingByBan() {
       //1. 단순 그룹화(반별로 그룹화)
        return personRepository
               .findByRole("student").stream()
               .collect(Collectors.groupingBy(Person::getBan));
    }
    public Map<Person.Level, List<Person>>  groupingByGrade() {
       //2. 단순그룹화(성적별로 그룹화)
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.groupingBy(
                      s->{
                          if(s.getScore()>=  80) return Person.Level.HIGH;
                          else  if(s.getScore()>=50)return Person.Level.MID;
                          else return  Person.Level.LOW;}
                      ));
       
    }
    public Map<Person.Level, Long>  groupingByCountByLevel() {
       //3. 단순그룹화 + 통계(성적별 학생수))
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.groupingBy(s->{
                  if(s.getScore()>= 80)  return Person.Level.HIGH;
                  else  if(s.getScore()>=50)return Person.Level.MID;
                  else                  return Person.Level.LOW;},
                      Collectors.counting()
               ));
               
               
    }
    public Map<Integer, Map<Integer,  List<Person>>> groupingByHakAndBan() {
       // 4. 다중 그룹화 (학년별, 반별)
       return  personRepository.findByRole("student").stream()
               .collect(
                   Collectors.groupingBy(Person::getHak,
                          Collectors.groupingBy(Person::getBan)
               ));
    }
    public Map<Integer, Map<Integer,  Object>> groupingTopByHakAndBan() {
       // 5. 다중 그룹화 +통계 (학년별, 반별  1등)
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.groupingBy(Person::getHak,
                      Collectors.groupingBy(Person::getBan,
                              Collectors.collectingAndThen(
                              Collectors.maxBy(Comparator
                                     .comparingInt(Person::getScore)),
                              Optional::get))));
    }
    public Map<Object, Set<Object>>  groupingByStat() {
       // 6. 다중그룹화 + 통계(학년별, 반별  성적그룹)
       return  personRepository.findByRole("student").stream()
               .collect(Collectors.groupingBy(s->  s.getHak()+"-"+ s.getBan(),
                      Collectors.mapping(s->{
                          if(s.getScore()>=  80) return Person.Level.HIGH;
                          else  if(s.getScore()>=50)return Person.Level.MID;
                          else                  return Person.Level.LOW;},
                          Collectors.toSet())));
                      
    }
}