练习2：用TreeMap类实现向数字图书馆中添加三本书籍，书籍类的属性有书籍编号，书籍名，作者，单价。遍历集合，按照书籍编号排序。（

要在Java中使用TreeMap类实现向数字图书馆添加书籍，并按书籍编号排序，首先需要创建一个Books类，包含书籍编号（bookId）、书籍名称（title）、作者（author）和单价（price）属性。然后，你可以按照以下步骤操作：

1. 定义Books类：

class Books {
    private int bookId;
    private String title;
    private String author;
    private double price;

    // 构造函数、getter和setter方法
    public Books(int bookId, String title, String author, double price) {
        this.bookId = bookId;
        this.title = title;
        this.author = author;
        this.price = price;
    }

    // getters and setters
}

2. 创建TreeMap实例，键为书籍编号，值为Books对象：

import java.util.TreeMap;

public class Library {
    private TreeMap<Integer, Books> bookCollection = new TreeMap<>();

    // 添加书籍方法
    public void addBook(Books book) {
        bookCollection.put(book.getBookId(), book);
    }
}

3. 向图书馆添加书籍：

Books book1 = new Books(1, "Book A", "Author A", 19.99);
Books book2 = new Books(2, "Book B", "Author B", 29.99);
Books book3 = new Books(3, "Book C", "Author C", 39.99);

Library library = new Library();
library.addBook(book1);
library.addBook(book2);
library.addBook(book3);

4. 遍历并打印书籍列表，按照书籍编号排序：

for (Map.Entry<Integer, Books> entry : library.bookCollection.entrySet()) {
    System.out.println("书籍编号: " + entry.getKey() +
                        ", 书名: " + entry.getValue().getTitle() +
                        ", 作者: " + entry.getValue().getAuthor() +
                        ", 单价: " + entry.getValue(). getPrice());
}