在Java中如何实现图书借阅记录统计

class Book {
    private String bookId;
    private String title;

    public Book(String bookId, String title) {
        this.bookId = bookId;
        this.title = title;
    }

    // getter 方法
    public String getBookId() { return bookId; }
    public String getTitle() { return title; }
}

class BorrowRecord {
    private String bookId;
    private String userId;
    private long borrowTime;
    private Long returnTime; // 未归还可为 null

    public BorrowRecord(String bookId, String userId, long borrowTime) {
        this.bookId = bookId;
        this.userId = userId;
        this.borrowTime = borrowTime;
        this.returnTime = null;
    }

    public void setReturnTime(Long returnTime) {
        this.returnTime = returnTime;
    }

    // getter 方法
    public String getBookId() { return bookId; }
    public String getUserId() { return userId; }
    public long getBorrowTime() { return borrowTime; }
    public Long getReturnTime() { return returnTime; }
    public boolean isReturned() { return returnTime != null; }
}

import java.util.*;

public class LibraryStats {
    private List records = new ArrayList<>();
    private Map books = new HashMap<>();

    public void addRecord(BorrowRecord record) {
        records.add(record);
    }

    public Map getBorrowCountByBook() {
        Map countMap = new HashMap<>();
        for (BorrowRecord record : records) {
            countMap.merge(record.getBookId(), 1, Integer::sum);
        }
        return countMap;
    }
}

public List getTopNBorrowedBooks(int n) {
    Map borrowCount = getBorrowCountByBook();

    return borrowCount.entrySet().stream()
        .sorted((a, b) -> b.getValue().compareTo(a.getValue()))
        .limit(n)
        .map(entry -> books.get(entry.getKey()))
        .filter(Objects::nonNull)
        .collect(Collectors.toList());
}

public Map getBorrowCountByUser() {
    Map userCount = new HashMap<>();
    for (BorrowRecord record : records) {
        userCount.merge(record.getUserId(), 1, Integer::sum);
    }
    return userCount;
}

public List getUsersWithOverdueBooks(long currentTime) {
    Map hasOverdue = new HashMap<>();
    for (BorrowRecord record : records) {
        if (!record.isReturned() && record.getBorrowTime() + 2_592_000_000L < currentTime) { // 假设借期30天
            hasOverdue.put(record.getUserId(), true);
        }
    }
    return hasOverdue.keySet().stream().collect(Collectors.toList());
}