Complete manual Spring Boot subject API

src/main/java/com/example/questionbank/manual/SubjectController.java

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+package com.example.questionbank.manual;
+
+import com.example.questionbank.manual.subject.CreateSubjectRequest;
+import com.example.questionbank.manual.subject.Subject;
+import com.example.questionbank.manual.subject.SubjectService;
+import jakarta.validation.Valid;
+import java.net.URI;
+import java.util.List;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+import org.springframework.web.server.ResponseStatusException;
+
+@RestController
+@RequestMapping("/api/subjects")
+public class SubjectController {
+
+    private final SubjectService subjectService;
+
+    public SubjectController(SubjectService subjectService) {
+        this.subjectService = subjectService;
+    }
+
+    @PostMapping
+    public ResponseEntity<Subject> create(
+            @Valid @RequestBody CreateSubjectRequest request
+    ) {
+        Subject created = subjectService.create(request.name());
+        return ResponseEntity
+                .created(URI.create("/api/subjects/" + created.id()))
+                .body(created);
+    }
+
+    @GetMapping
+    public List<Subject> findAll() {
+        return subjectService.findAll();
+    }
+
+    @GetMapping("/{id}")
+    public Subject findById(@PathVariable Long id) {
+        return subjectService.findById(id)
+                .orElseThrow(() -> new ResponseStatusException(
+                        HttpStatus.NOT_FOUND,
+                        "과목을 찾을 수 없습니다. id=" + id
+                ));
+    }
+}

src/main/java/com/example/questionbank/manual/subject/CreateSubjectRequest.java

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+package com.example.questionbank.manual.subject;
+
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.Size;
+
+public record CreateSubjectRequest(
+        @NotBlank(message = "과목명은 필수입니다.")
+        @Size(max = 100, message = "과목명은 100자 이하여야 합니다.")
+        String name
+) {
+}

src/main/java/com/example/questionbank/manual/subject/Subject.java

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+package com.example.questionbank.manual.subject;
+
+public record Subject(Long id, String name) {
+
+}

src/main/java/com/example/questionbank/manual/subject/SubjectService.java

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+package com.example.questionbank.manual.subject;
+
+import java.util.Comparator;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.atomic.AtomicLong;
+import org.springframework.stereotype.Service;
+
+@Service
+public class SubjectService {
+
+    private final Map<Long, Subject> subjects = new ConcurrentHashMap<>();
+    private final AtomicLong sequence = new AtomicLong();
+
+    public Subject create(String name) {
+        long id = sequence.incrementAndGet();
+        Subject subject = new Subject(id, name.trim());
+        subjects.put(id, subject);
+        return subject;
+    }
+
+    public List<Subject> findAll() {
+        return subjects.values()
+                .stream()
+                .sorted(Comparator.comparing(Subject::id))
+                .toList();
+    }
+
+    public Optional<Subject> findById(Long id) {
+        return Optional.ofNullable(subjects.get(id));
+    }
+}