如何用Java实现图书分类浏览功能

public class Category {
    private Long id;
    private String name;
    private Long parentId; // 父分类ID，根分类为null或0
    private List children = new ArrayList<>();
    // 构造方法、getter/setter省略
}

public class Book {
    private Long id;
    private String title;
    private String author;
    private Long categoryId; // 所属分类ID
    // getter/setter省略
}

public List buildCategoryTree(List categories) {
    Map categoryMap = new HashMap<>();
    List rootCategories = new ArrayList<>();

    // 先将所有分类放入map，便于查找
    for (Category c : categories) {
        categoryMap.put(c.getId(), c);
    }

    // 遍历，建立父子关系
    for (Category c : categories) {
        if (c.getParentId() == null || c.getParentId() == 0) {
            rootCategories.add(c);
        } else {
            Category parent = categoryMap.get(c.getParentId());
            if (parent != null) {
                parent.getChildren().add(c);
            }
        }
    }
    return rootCategories;
}

// 查询所有分类
String sql = "SELECT id, name, parent_id FROM categories";
List categories = new ArrayList<>();
try (Statement stmt = connection.createStatement();
     ResultSet rs = stmt.executeQuery(sql)) {
    while (rs.next()) {
        Category c = new Category();
        c.setId(rs.getLong("id"));
        c.setName(rs.getString("name"));
        c.setParentId(rs.getLong("parent_id"));
        categories.add(c);
    }
}
List tree = buildCategoryTree(categories);

String sql = "SELECT id, title, author FROM books WHERE categoryId = ?";
try (PreparedStatement pstmt = connection.prepareStatement(sql)) {
    pstmt.setLong(1, categoryId);
    ResultSet rs = pstmt.executeQuery();
    List books = new ArrayList<>();
    while (rs.next()) {
        Book book = new Book();
        book.setId(rs.getLong("id"));
        book.setTitle(rs.getString("title"));
        book.setAuthor(rs.getString("author"));
        books.add(book);
    }
}

@RestController
@RequestMapping("/api/categories")
public class CategoryController {

    @Autowired
    private CategoryService categoryService;

    @GetMapping
    public List getCategoryTree() {
        List all = categoryService.getAllCategories();
        return categoryService.buildCategoryTree(all);
    }

    @GetMapping("/{categoryId}/books")
    public List getBooksByCategory(@PathVariable Long categoryId) {
        return bookService.findByCategoryId(categoryId);
    }
}