java实现图书查看部分，先获取key，根据key遍历所有集合value，遍历每个类目下的集合value的代码

以下是一份示例代码，可以作为参考：

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class BookCatalog {
    private Map<String, Set<Book>> catalog;

    public BookCatalog(Map<String, Set<Book>> catalog) {
        this.catalog = catalog;
    }

    public Set<Book> search(String key) {
        if (!catalog.containsKey(key)) {
            return null;
        }

        return catalog.get(key);
    }

    public static void main(String[] args) {
        // 示例用的数据
        Map<String, Set<Book>> catalog = new HashMap<>();

        Set<Book> programmingBooks = Set.of(
                new Book("Effective Java", "Joshua Bloch", 2008),
                new Book("Clean Code", "Robert Martin", 2008)
        );

        Set<Book> fictionBooks = Set.of(
                new Book("To Kill a Mockingbird", "Harper Lee", 1960),
                new Book("1984", "George Orwell", 1949)
        );

        catalog.put("programming", programmingBooks);
        catalog.put("fiction", fictionBooks);

        // 使用示例
        BookCatalog bookCatalog = new BookCatalog(catalog);

        Set<Book> result = bookCatalog.search("programming");
        if (result != null) {
            for (Book book : result) {
                System.out.println(book);
            }
        } else {
            System.out.println("No results.");
        }
    }
}

class Book {
    private String title;
    private String author;
    private int year;

    public Book(String title, String author, int year) {
        this.title = title;
        this.author = author;
        this.year = year;
    }

    public String getTitle() {
        return title;
    }

    public String getAuthor() {
        return author;
    }

    public int getYear() {
        return year;
    }

    @Override
    public String toString() {
        return title + " by " + author + " (" + year + ")";
    }
}