feat : Member Service function add

src/main/java/hello/hellospring/service/MemberService.java

@@ -0,0 +1,35 @@
+package hello.hellospring.service;
+
+import java.util.List;
+import java.util.Optional;
+
+import hello.hellospring.domain.Member;
+import hello.hellospring.repository.MemberReository;
+import hello.hellospring.repository.MemoryMemberRepository;
+
+public class MemberService {
+
+	private final MemberReository memberReository = new MemoryMemberRepository();
+
+	public Long join(Member member) {
+		validateDuplicateMember(member);
+		memberReository.save(member);
+		return member.getId();
+	}
+
+	public void validateDuplicateMember(Member member) {
+		memberReository.findByName(member.getName())
+			.ifPresent(m -> {
+				throw new IllegalStateException("이미 존재하는 회원입니다.");
+			});
+	}
+
+	public List<Member> findMember() {
+		return memberReository.findAll();
+	}
+
+	public Optional<Member> findOne(Long memberId) {
+		return memberReository.findById(memberId);
+	}
+
+}