// Step 1: 抽出全部相邻对
public static String[] extractPairs(String text) {
    int n = text.length();
    if (n < 2) return new String[0];          // 边界：空串或单字符
    String[] pairs = new String[n - 1];
    for (int i = 0; i < n - 1; i++) {
        pairs[i] = text.substring(i, i + 2);
    }
    return pairs;
}

// Step 2: 统计每个唯一对的出现次数，返回唯一对个数
public static int countNumber(String[] pairs, String[] uniquePairs, int[] counts) {
    int uniqueCount = 0;
    for (String p : pairs) {
        int idx = -1;
        for (int i = 0; i < uniqueCount; i++) {       // 在已记录的唯一对里找
            if (uniquePairs[i].equals(p)) { idx = i; break; }
        }
        if (idx == -1) {                               // 新出现的对
            uniquePairs[uniqueCount] = p;
            counts[uniqueCount] = 1;
            uniqueCount++;
        } else {
            counts[idx]++;
        }
    }
    return uniqueCount;
}

// Step 3: 找出现次数最多的对
public static String getMostFrequentPair(String[] uniquePairs, int[] counts, int uniqueCount) {
    if (uniqueCount == 0) return null;
    int best = 0;
    for (int i = 1; i < uniqueCount; i++) {
        if (counts[i] > counts[best]) best = i;        // 严格大于：并列时取先出现的
    }
    return uniquePairs[best];
}

// Step 4: 替换
public static String replacePair(String text, String pair, String symbol) {
    return text.replace(pair, symbol);
}

运行结果：Original text: aaabdaaabac ／ Most frequent pair: aa ／ New text: zabdzabac。

private static string[] GetRecord(string inId) {
    for (int r = 0; r < AllRecords.GetLength(0); r++) {
        if (AllRecords[r, 0] == inId) {
            string[] row = new string[AllRecords.GetLength(1)];
            for (int c = 0; c < AllRecords.GetLength(1); c++) row[c] = AllRecords[r, c];
            return row;
        }
    }
    return null;                       // 找不到
}

private static bool WasAtTheSameLocation(string candLoc, string targetLoc) {
    return string.Equals(candLoc, targetLoc, StringComparison.OrdinalIgnoreCase);
}

private static bool DidActivityOverlap(DateTime cEntry, DateTime cExit,
                                       DateTime tEntry, DateTime tExit) {
    return cEntry < tExit && cExit > tEntry;   // 交集非空
}

private static void PrintLinkedIds(string targetId, DateTime tEntry,
                                   DateTime tExit, string tLocation) {
    for (int r = 0; r < AllRecords.GetLength(0); r++) {
        string id = AllRecords[r, 0];
        if (id == targetId) continue;                       // 跳过自己
        string loc = AllRecords[r, 4];
        DateTime cEntry = GetTimeFromTimeStampString(AllRecords[r, 2]);
        DateTime cExit  = GetTimeFromTimeStampString(AllRecords[r, 3]);
        if (WasAtTheSameLocation(loc, tLocation) && DidActivityOverlap(cEntry, cExit, tEntry, tExit)) {
            Console.WriteLine(id + " is linked with " + targetId);
        }
    }
}

输出格式要与 Figure 2 一致：11236375 is linked with 11235929。

public static int mostFrequent(int[] values) {
    int bestVal = values[0], bestCount = 0;
    for (int i = 0; i < values.length; i++) {
        int c = 0;
        for (int j = 0; j < values.length; j++) if (values[j] == values[i]) c++;
        if (c > bestCount || (c == bestCount && values[i] < bestVal)) {
            bestCount = c; bestVal = values[i];
        }
    }
    return bestVal;
}

O(n²) 在这类考试里完全可以接受；用 HashMap 也可以，但要多写 5 行且容易在并列规则上出错。

public static String compress(String s) {
    if (s == null || s.isEmpty()) return s;
    StringBuilder sb = new StringBuilder();
    int count = 1;
    for (int i = 1; i <= s.length(); i++) {
        if (i < s.length() && s.charAt(i) == s.charAt(i - 1)) {
            count++;
        } else {
            sb.append(s.charAt(i - 1)).append(count);
            count = 1;
        }
    }
    return sb.length() < s.length() ? sb.toString() : s;
}

关键在 i <= s.length()：多跑一格才能把最后一组收尾，否则会漏掉末尾。

public static long overlapHours(LocalDateTime aStart, LocalDateTime aEnd,
                                LocalDateTime bStart, LocalDateTime bEnd) {
    LocalDateTime start = aStart.isAfter(bStart) ? aStart : bStart;   // 较晚的开始
    LocalDateTime end   = aEnd.isBefore(bEnd)  ? aEnd   : bEnd;       // 较早的结束
    if (!start.isBefore(end)) return 0;                               // 无交集
    return java.time.Duration.between(start, end).toHours();
}

与样卷密接题的判据同源：交集 = max(开始) < min(结束)。差别只是这里要算长度。

(a) CourseName。理由三条（写两条即可）：选择性高（课程名几乎唯一，索引能把候选行压到 1 行）；查询频繁（第 (b)(e) 问都按名称过滤）；更新频率低。反例：Credits 只有 2/3/4 三种取值，选择性低，建索引帮助有限。

-- (b)
SELECT s.StudentId, s.StudentName, c.CourseName
FROM   Student s
JOIN   Enrolment e  ON e.StudentId = s.StudentId
JOIN   Course c     ON c.CourseId  = e.CourseId
JOIN   Instructor i ON i.InstructorId = c.InstructorId
WHERE  i.InstructorName = 'Dr. Smith'
ORDER  BY s.StudentName DESC;

-- (c)
SELECT c.CourseId, c.CourseName, c.InstructorId, COUNT(*) AS A_Students
FROM   Course c
JOIN   Enrolment e ON e.CourseId = c.CourseId
WHERE  e.Grade = 'A'
GROUP  BY c.CourseId, c.CourseName, c.InstructorId
HAVING COUNT(*) >= 2;

-- (d) 自连接：c 是课程，p 是它的先修课；e1/e2 分别修两门课
SELECT c.CourseName,
       p.CourseName AS PrerequisiteCourseName,
       COUNT(DISTINCT e1.StudentId) AS StudentsEnrolledInBothCourses
FROM   Course c
JOIN   Course p     ON p.CourseId = c.PrerequisiteId
JOIN   Enrolment e1 ON e1.CourseId = c.CourseId
JOIN   Enrolment e2 ON e2.CourseId = p.CourseId AND e2.StudentId = e1.StudentId
GROUP  BY c.CourseName, p.CourseName;

-- (e)
CREATE VIEW StudentCourseDetails AS
SELECT s.StudentId, s.StudentName, c.CourseId, c.CourseName, e.EnrolDate
FROM   Student s
JOIN   Enrolment e ON e.StudentId = s.StudentId
JOIN   Course c    ON c.CourseId  = e.CourseId;

SELECT * FROM StudentCourseDetails WHERE CourseName = 'Database Systems';

-- (a)
SELECT d.Name
FROM   Employee e
JOIN   Department d ON d.DepartmentID = e.DepartmentID
WHERE  e.Name = 'Jenny Lawrence';

-- (b) 孤儿记录：左连接后右表为空
SELECT e.EmployeeID, e.Name
FROM   Employee e
LEFT   JOIN Department d ON d.DepartmentID = e.DepartmentID
WHERE  d.DepartmentID IS NULL;

-- (c) ALL 语义 + 人数统计
SELECT d.Name
FROM   Department d
JOIN   DepartmentProject dp ON dp.DepartmentId = d.DepartmentID
JOIN   Employee e           ON e.DepartmentID = d.DepartmentID
GROUP  BY d.DepartmentID, d.Name
HAVING MIN(dp.DepProjBudget) > 1000000
   AND COUNT(DISTINCT e.EmployeeID) > 5;

-- (d)
CREATE PROCEDURE usp_UpdateConsultRate
    @EmployeeID varchar(6),
    @NewRate    int
AS
BEGIN
    UPDATE Employee
    SET    DailyConsultRate = @NewRate
    WHERE  EmployeeID = @EmployeeID;
END;
GO
-- 测试
EXEC usp_UpdateConsultRate 'E001', 750;
SELECT EmployeeID, Name, DailyConsultRate FROM Employee WHERE EmployeeID = 'E001';

-- (e)
CREATE VIEW DepartmentEmployeeView AS
SELECT d.Name AS DepartmentName,
       e.Name AS EmployeeName,
       e.DailyConsultRate
FROM   Employee e
JOIN   Department d ON d.DepartmentID = e.DepartmentID;

SELECT EmployeeName FROM DepartmentEmployeeView WHERE DailyConsultRate > 500;

-- 1) 窗口函数排名（MS SQL）
WITH CourseCount AS (
    SELECT i.InstructorId, i.InstructorName, i.Department,
           COUNT(c.CourseId) AS CourseCount,
           ROW_NUMBER() OVER (PARTITION BY i.Department
                              ORDER BY COUNT(c.CourseId) DESC) AS rn
    FROM   Instructor i
    LEFT   JOIN Course c ON c.InstructorId = i.InstructorId
    GROUP  BY i.InstructorId, i.InstructorName, i.Department
)
SELECT Department, InstructorName, CourseCount
FROM   CourseCount
WHERE  rn = 1;

-- 2) 从未选课的学生
SELECT s.StudentId, s.StudentName
FROM   Student s
WHERE  NOT EXISTS (SELECT 1 FROM Enrolment e WHERE e.StudentId = s.StudentId);

-- 3) CTE 计算平均绩点
WITH GradePoint AS (
    SELECT CourseId,
           AVG(CASE Grade WHEN 'A' THEN 4 WHEN 'B' THEN 3
                          WHEN 'C' THEN 2 WHEN 'D' THEN 1
                          WHEN 'F' THEN 0 END) AS AvgPoint
    FROM   Enrolment
    WHERE  Grade IS NOT NULL
    GROUP  BY CourseId
)
SELECT c.CourseId, c.CourseName, gp.AvgPoint
FROM   GradePoint gp
JOIN   Course c ON c.CourseId = gp.CourseId
WHERE  gp.AvgPoint < 2.5
ORDER  BY gp.AvgPoint;

注意 AVG 会忽略 NULL，但 CASE 遇到 NULL 也返回 NULL，所以要显式写 WHERE Grade IS NOT NULL，否则分母对但语义含糊。

// (a) 两个子类
public class SalariedEmployee extends Employee {
    private double monthlySalary;

    public SalariedEmployee(String name, String employeeId, double monthlySalary) {
        super(name, employeeId);
        this.monthlySalary = monthlySalary;
    }
    @Override public String getEmployeeType() { return "Salaried"; }

    // (d)
    @Override public double calPaymentAmt() { return monthlySalary; }
}

public class HourlyEmployee extends Employee {
    private double hourlyRate;
    private int hoursWorked;

    public HourlyEmployee(String name, String employeeId, double hourlyRate, int hoursWorked) {
        super(name, employeeId);
        this.hourlyRate = hourlyRate;
        this.hoursWorked = hoursWorked;
    }
    @Override public String getEmployeeType() { return "Hourly"; }

    // (d)
    @Override public double calPaymentAmt() { return hourlyRate * hoursWorked; }
}

// (b) 父类实现接口，仍保持 abstract
public abstract class Employee implements IPayable {
    private String name;
    private String employeeId;

    public Employee(String name, String employeeId) {
        this.name = name;
        this.employeeId = employeeId;
    }
    public abstract String getEmployeeType();

    // (c) 三段信息都在父类，写在这里，子类不用重复
    @Override
    public String getPaymentDetails() {
        return String.format("Employee (%s): %s, ID: %s",
                             getEmployeeType(), name, employeeId);
    }
}

为什么能编过：Employee 声明实现 IPayable，只实现了 getPaymentDetails()，calPaymentAmt() 留给子类——抽象类允许有未实现的接口方法。

public class Person {
    private String email;
    private String name;
    private int yearOfBirth;          // 0 表示未提供

    public Person(String email, String name, int yearOfBirth) {
        this.email = email; this.name = name; this.yearOfBirth = yearOfBirth;
    }
    public Person(String email, String name) { this(email, name, 0); }   // 委托，避免重复

    public String getEmail() { return email; }
    public String getName() { return name; }
    public int getYearOfBirth() { return yearOfBirth; }
    public void setEmail(String e) { this.email = e; }
    public void setName(String n) { this.name = n; }
    public void setYearOfBirth(int y) { this.yearOfBirth = y; }

    @Override
    public String toString() {
        String base = "Email: " + email + ", Name: " + name;
        return yearOfBirth > 0 ? base + ", Year of Birth: " + yearOfBirth : base;
    }

    public void Display() { System.out.println(this); }   // 依赖 toString，子类自动适配
}

public class Employee extends Person {
    private double salary;
    public Employee(String email, String name, int yob, double salary) {
        super(email, name, yob);
        this.salary = salary;
    }
    public double getSalary() { return salary; }
    public void setSalary(double s) { this.salary = s; }

    @Override
    public String toString() { return super.toString() + ", Salary: " + salary; }
}

public class Faculty extends Employee {
    private boolean tenured;
    public Faculty(String email, String name, int yob, double salary, boolean tenured) {
        super(email, name, yob, salary);
        this.tenured = tenured;
    }
    @Override
    public String toString() { return super.toString() + ", Is Tenure: " + tenured; }
}

public class Student extends Person {
    private String major;
    public Student(String email, String name, int yob, String major) {
        super(email, name, yob);
        this.major = major;
    }
    @Override
    public String toString() { return super.toString() + ", Major: " + major; }
}

public class PersonelRecords {
    private final List<Person> people = new ArrayList<>();

    public void Add(Person p) { people.add(p); }

    public void Display() {
        for (Person p : people) {
            System.out.println(p);        // 多态：调用各自的 toString()
        }
    }
}

输出大小写要对：Is Tenure: true 还是 True？题面样例是 True（首字母大写），如果你的输出是 true，判分时会扣“格式”分——这是这道题最阴的坑。

public abstract class Shape {
    protected String name;
    protected Shape(String name) { this.name = name; }

    public abstract double area();

    public String describe() {
        return String.format("%s with area %.2f", name, area());
    }
}

public interface IResizable {
    void resize(double factor);
}

public class Circle extends Shape implements IResizable {
    private double radius;
    public Circle(double radius) { super("Circle"); this.radius = radius; }
    @Override public double area() { return Math.PI * radius * radius; }
    @Override public void resize(double factor) { radius *= factor; }
}

public class Rectangle extends Shape implements IResizable {
    private double width, height;
    public Rectangle(double w, double h) { super("Rectangle"); this.width = w; this.height = h; }
    @Override public double area() { return width * height; }
    @Override public void resize(double factor) { width *= factor; height *= factor; }
}

public class ShapeRegistry {
    private final List<Shape> shapes = new ArrayList<>();
    public void add(Shape s) { shapes.add(s); }

    public void scaleAllAndPrint(double factor) {
        for (Shape s : shapes) {
            if (s instanceof IResizable r) r.resize(factor);   // 只有可缩放的才缩
            System.out.println(s.describe());
        }
    }
}

注意 instanceof IResizable 这一手：ShapeRegistry 里放的是 Shape，不是所有 Shape 都可缩放——这正是接口存在的意义。Java 16+ 支持模式匹配写法，写成传统 if (s instanceof IResizable) { ((IResizable) s).resize(factor); } 也完全给分。

@Entity
public class Author {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String name;
    private int yearOfBirth;
    private String nationality;

    @ManyToMany(mappedBy = "authors")          // 由 Book 维护中间表
    private List<Book> books = new ArrayList<>();

    public double getAverageRating() {          // 各书均分的平均
        if (books.isEmpty()) return 0.0;
        double sum = 0;
        for (Book b : books) sum += b.getAverageRating();
        return sum / books.size();
    }
    // getters / setters 略（题目不要求写）
}

@Entity
public class Book {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String title;
    private int publishedYear;
    private String isbn;

    @ManyToMany
    @JoinTable(name = "book_author",
        joinColumns = @JoinColumn(name = "book_id"),
        inverseJoinColumns = @JoinColumn(name = "author_id"))
    private List<Author> authors = new ArrayList<>();

    @OneToMany(mappedBy = "book", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Review> reviews = new ArrayList<>();

    public double getAverageRating() {
        if (reviews.isEmpty()) return 0.0;
        int sum = 0;
        for (Review r : reviews) sum += r.getRating();
        return (double) sum / reviews.size();
    }
    public int getReviewCount() { return reviews.size(); }
}

@Entity
public class Review {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @ManyToOne
    @JoinColumn(name = "book_id", nullable = false)
    private Book book;

    private int rating;                 // 1..5
    @Column(length = 1000)
    private String content;
}

答案 (b)：Repository

public interface AuthorRepository extends JpaRepository<Author, Long> {

    // 一次抓取作者 + 书 + 评论，避免 N+1 与 LazyInitializationException
    @Query("select distinct a from Author a " +
           "left join fetch a.books b " +
           "left join fetch b.reviews")
    List<Author> findAllWithBooksAndReviews();
}

public interface ReviewRepository extends JpaRepository<Review, Long> { }

答案 (c)：Controller

@Controller
public class AuthorController {

    private final AuthorRepository authorRepository;
    public AuthorController(AuthorRepository authorRepository) {
        this.authorRepository = authorRepository;
    }

    @GetMapping("/authors")
    public String list(Model model) {
        model.addAttribute("authors", authorRepository.findAllWithBooksAndReviews());
        return "authors";                     // → templates/authors.html
    }
}

答案 (d)：Thymeleaf

<!DOCTYPE html>
<html xmlns:th="http://www.thymeleaf.org">
<head><title>Authors</title></head>
<body>
<h1>Authors and Books</h1>
<ul>
  <li th:each="a : ${authors}">
    <b th:text="${a.name}">Name</b>
    (<span th:text="${a.nationality}">Nationality</span> author,
     born <span th:text="${a.yearOfBirth}">1900</span>)
    <ul>
      <li th:each="b : ${a.books}">
        Book Title: <b th:text="${b.title}">Title</b><br>
        Published Year: <span th:text="${b.publishedYear}">1949</span><br>
        ISBN: <span th:text="${b.isbn}">978</span><br>
        Number of Reviews: <span th:text="${b.reviewCount}">0</span><br>
        Average Rating: <span th:text="${#numbers.formatDecimal(b.averageRating,1,1)}">0.0</span>
      </li>
    </ul>
    Author's Average Rating:
    <b th:text="${#numbers.formatDecimal(a.averageRating,1,1)}">0.0</b>
  </li>
</ul>
</body>
</html>

reviewCount 与 averageRating 都是实体里的 getter，模板里不写运算逻辑——这是这道题最省时也最不容易错的写法。

答案 (e)：REST

@RestController
@RequestMapping("/api/review")
public class ReviewRestController {

    private final BookRepository bookRepository;
    private final ReviewRepository reviewRepository;
    public ReviewRestController(BookRepository b, ReviewRepository r) {
        this.bookRepository = b; this.reviewRepository = r;
    }

    @PostMapping("/{bookId}")
    public ResponseEntity<Void> create(@PathVariable Long bookId,
                                      @RequestBody ReviewRequest body) {
        Optional<Book> book = bookRepository.findById(bookId);
        if (book.isEmpty()) {
            return ResponseEntity.status(HttpStatus.UNPROCESSABLE_ENTITY).build();  // 422
        }
        Review review = new Review();
        review.setBook(book.get());
        review.setRating(body.rating());
        review.setContent(body.content());
        reviewRepository.save(review);
        return ResponseEntity.status(HttpStatus.CREATED).build();                  // 201
    }

    public record ReviewRequest(int rating, String content) { }
}

@Entity
public class Member {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long memberId;
    private String name;

    @OneToMany(mappedBy = "member", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Address> address = new ArrayList<>();

    @OneToMany(mappedBy = "member")
    private List<Order> orders = new ArrayList<>();
}

@Entity
public class Address {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long addressId;
    private String line1, city, postalCode;

    @ManyToOne
    @JoinColumn(name = "member_id")
    private Member member;
}

@Entity
@Table(name = "orders")          // ORDER 是 SQL 保留字
public class Order {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long orderId;
    private Double amount;
    @Temporal(TemporalType.DATE) private Date orderdate;
    private Long orderNum;

    @ManyToOne @JoinColumn(name = "member_id")   private Member member;
    @ManyToOne @JoinColumn(name = "address_id")  private Address address;
}

答案 (Q4b)：Top 10 高消费会员

public interface OrderRepository extends JpaRepository<Order, Long> {

    @Query("select o.member from Order o " +
           "group by o.member " +
           "having sum(o.amount) > 100000 " +
           "order by sum(o.amount) desc")
    List<Member> findTopSpenders(Pageable pageable);
    // 调用：findTopSpenders(PageRequest.of(0, 10))
}

答案 (Q4c)：动态条件查询

@Override
public List<Order> findAll(OrderSearch s) {
    Specification<Order> spec = (root, query, cb) -> {
        List<Predicate> ps = new ArrayList<>();
        if (s.getOrderStatus() != null)
            ps.add(cb.equal(root.get("status"), s.getOrderStatus()));
        if (s.getOrderDate() != null)
            ps.add(cb.greaterThanOrEqualTo(root.get("orderdate"), s.getOrderDate()));
        if (s.getMemberName() != null && !s.getMemberName().isBlank())
            ps.add(cb.like(cb.lower(root.get("member").get("name")),
                           "%" + s.getMemberName().toLowerCase() + "%"));
        return cb.and(ps.toArray(new Predicate[0]));
    };
    return orderRepository.findAll(spec);
}
// 也可以直接用派生查询：
// List<Order> findByMemberNameContainingIgnoreCase(String name);
// List<Order> findByStatus(OrderStatus status);

答案 (Q4d)：列表 Controller

@GetMapping("/items")
public String list(Model model) {
    model.addAttribute("items", itemRepository.findAll());
    return "items";            // templates/items.html（对应 Figure 4 Items Listing）
}

答案 (Q4e)：表单 HTML

<form th:action="@{/items}" method="post" th:object="${item}">
    <label>Name</label>
    <input type="text" th:field="*{name}" required>

    <label>Price</label>
    <input type="number" step="0.01" th:field="*{price}" required>

    <label>Category</label>
    <select th:field="*{category}">
        <option th:each="c : ${categories}"
                th:value="${c.id}" th:text="${c.name}"></option>
    </select>

    <button type="submit">Add Item</button>
</form>

答案 (Q5a)：校验分层

Validation is necessary: price and quantity must be positive and the name is required, otherwise invalid data reaches the database. The most suitable components are Bean Validation annotations on the entity or a form-backing DTO (@NotBlank, @Positive, @Size), enforced in the controller with @Valid plus BindingResult so the form is re-rendered with error messages; business-level rules (e.g. “item must belong to an existing category”) belong to the service layer. Client-side required/min attributes are only a convenience and cannot replace server-side validation.

答案 (Q5b)：单元测试

@DataJpaTest
class MemberRepositoryTest {

    @Autowired MemberRepository memberRepository;

    @Test
    void createsMember() {
        Member m = new Member();
        m.setName("Alice");
        Member saved = memberRepository.save(m);

        assertNotNull(saved.getMemberId());
        assertEquals("Alice", memberRepository.findById(saved.getMemberId()).orElseThrow().getName());
    }
}

// 1)
@PutMapping("/{id}")
public ResponseEntity<Book> update(@PathVariable Long id, @RequestBody Book body) {
    return bookRepository.findById(id)
        .map(existing -> {
            existing.setTitle(body.getTitle());
            existing.setAuthor(body.getAuthor());
            return ResponseEntity.ok(bookRepository.save(existing));      // 200
        })
        .orElseGet(() -> ResponseEntity.notFound().build());               // 404
}

// 2) 实体里加一个方法
public Map<Integer, Long> getRatingDistribution() {
    return reviews.stream().collect(
        Collectors.groupingBy(Review::getRating, Collectors.counting()));
}
// 模板
<ul>
  <li th:each="e : ${book.ratingDistribution}">
    <span th:text="${e.key}">5</span>-star:
    <span th:text="${e.value}">2</span>
  </li>
</ul>

// 3)
@WebMvcTest(BookController.class)
class BookControllerTest {
    @Autowired MockMvc mvc;
    @MockBean BookService bookService;

    @Test
    void missingBookReturns404() throws Exception {
        when(bookService.findById(999L)).thenReturn(null);
        mvc.perform(get("/books/999"))
           .andExpect(status().isNotFound());
    }
}