MemberService.java

package hello.hellospring.service;

import hello.hellospring.domain.Member;
import hello.hellospring.repository.MemberRepository;
import hello.hellospring.repository.MemoryMemberRepository;

import java.util.List;
import java.util.Optional;

public class MemberService {

    private final MemberRepository memberRepository;

    public MemberService(MemberRepository memberRepository){
        this.memberRepository = memberRepository;
    }
    /**
     * 회원 가입
     */
    public Long join(Member member) {
        validateDuplicateMember(member);

        memberRepository.save(member);
        return member.getId();
    }

    private void validateDuplicateMember(Member member) {
        // 같은 이름이 있는 중복 회원 X
        memberRepository.findByName(member.getName()) // Optional member를 반환
                .ifPresent(m -> { // ifPresent: 값이 있으면 수행
                    throw new IllegalStateException("이미 존재하는 회원입니다."); // 에러 발생
                });
    }

    /**
     * 전체 회원 조회
     */
    public List<Member> findMembers(){
        return memberRepository.findAll();
    }

    public  Optional<Member> findOne(Long memberId){
        return memberRepository.findById(memberId);

    }
}