java编写了一个图书商店系统，但是缺少顾客购买书籍并保存的方法与展示顾客购买书籍信息的方法，现如今需要补全代码

在Java中，如果你正在构建一个图书商店系统，你可以创建一个`Customer`类和一个`Book`类来表示这个场景。首先，我们可以定义一个`Purchase`类来表示购买行为，然后在`Customer`类中添加购买和查看购买历史的方法。这是一个简单的示例：

// 定义Book类
class Book {
    String title;
    double price;

    // 构造函数， getters 和 setters 省略
}

// 定义Purchase类
class Purchase {
    private Customer customer;
    private Book book;
    private Date purchaseDate;

    public Purchase(Customer customer, Book book) {
        this.customer = customer;
        this.book = book;
        this.purchaseDate = new Date();
    }

    // 添加保存购买记录的方法
    public void savePurchase() {
        // 假设这里有一个数据库操作，实际项目中会连接到数据库并保存数据
        System.out.println("Purchase saved for " + customer.getName() + " on " + purchaseDate);
    }
    
    // 添加获取购买历史的方法，返回Purchase列表
    public List<Purchase> getPurchaseHistory() {
        // 实际项目中应从数据库查询，这里仅模拟一个空列表
        return Collections.emptyList();
    }
}

// 定义Customer类
class Customer {
    private String name;
    private List<Purchase> purchaseHistory;

    public Customer(String name) {
        this.name = name;
        this.purchaseHistory = new ArrayList<>();
    }

    // 购买书籍的方法
    public void buyBook(Book book) {
        if (book != null) {
            Purchase purchase = new Purchase(this, book);
            purchase.savePurchase();
            purchaseHistory.add(purchase);
        } else {
            throw new IllegalArgumentException("Book cannot be null.");
        }
    }

    // 展示购买书籍信息的方法
    public void showPurchaseHistory() {
        if (!purchaseHistory.isEmpty()) {
            System.out.println("Purchase history for " + name + ":");
            for (Purchase p : purchaseHistory) {
                System.out.println("- " + p.getBook().getTitle() + " - Price: " + p.getBook().price + " - Date: " + p.purchaseDate);
            }
        } else {
            System.out.println(name + " has no purchase history yet.");
        }
    }
}

// 使用示例
public class Main {
    public static void main(String[] args) {
        Customer customer = new Customer("Alice");
        Book book1 = new Book("The Great Gatsby", 10.99);
        Book book2 = new Book("To Kill a Mockingbird", 12.45);

        customer.buyBook(book1);
        customer.buyBook(book2);
        
        customer.showPurchaseHistory();
    }
}