Java中按日期分组对象列表的实用教程

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class Payment {
    private int paymentID;
    private String paymentDate; // 存储为字符串，例如 "YYYY-MM-DD"
    private int paymentTotal;

    public Payment(int paymentID, String paymentDate, int paymentTotal) {
        this.paymentID = paymentID;
        this.paymentDate = paymentDate;
        this.paymentTotal = paymentTotal;
    }

    public int getPaymentID() {
        return paymentID;
    }

    public String getPaymentDate() {
        return paymentDate;
    }

    public int getPaymentTotal() {
        return paymentTotal;
    }

    @Override
    public String toString() {
        return "Payment{" +
               "paymentID=" + paymentID +
               ", paymentDate='" + paymentDate + '\'' +
               ", paymentTotal=" + paymentTotal +
               '}';
    }
}

public class PaymentGrouper {

    // 定义日期格式化器，与 paymentDate 字符串格式匹配
    private static final DateTimeFormatter DATE_FORMATTER = DateTimeFormatter.ofPattern("yyyy-MM-dd");

    /**
     * 将支付列表按日期分组。
     *
     * @param allPayments 从数据库获取的所有支付记录列表
     * @return 一个列表的列表，每个内部列表包含同一天的支付记录
     */
    public List<List<Payment>> groupPaymentsByDate(List<Payment> allPayments) {
        // 使用 HashMap 存储按日期分组的支付记录
        Map<LocalDate, List<Payment>> paymentsByDateMap = new HashMap<>();

        for (Payment payment : allPayments) {
            // 解析 paymentDate 字符串为 LocalDate
            LocalDate date;
            try {
                date = LocalDate.parse(payment.getPaymentDate(), DATE_FORMATTER);
            } catch (java.time.format.DateTimeParseException e) {
                System.err.println("Error parsing date for payment ID " + payment.getPaymentID() +
                                   ": " + payment.getPaymentDate() + ". Skipping this payment.");
                // 可以选择抛出异常或采取其他错误处理措施
                continue; // 跳过当前支付记录，处理下一个
            }

            // 如果 Map 中还没有这个日期的键，则创建一个新的列表
            // computeIfAbsent 是一个更简洁的 Java 8+ 方式
            paymentsByDateMap.computeIfAbsent(date, k -> new ArrayList<>()).add(payment);

            /*
            // 传统方式：
            List<Payment> dailyPayments = paymentsByDateMap.get(date);
            if (dailyPayments == null) {
                dailyPayments = new ArrayList<>();
                paymentsByDateMap.put(date, dailyPayments);
            }
            dailyPayments.add(payment);
            */
        }

        // 将 Map 中的所有值（即按日期分组的列表）收集到一个新的 List 中
        // 如果需要按日期排序，可以在这里对 paymentsByDateMap 的 keySet 进行排序，然后遍历
        List<List<Payment>> groupedLists = new ArrayList<>(paymentsByDateMap.values());

        // 如果需要按日期对外部列表进行排序，可以这样做：
        // Collections.sort(groupedLists, (list1, list2) -> {
        //     LocalDate date1 = LocalDate.parse(list1.get(0).getPaymentDate(), DATE_FORMATTER);
        //     LocalDate date2 = LocalDate.parse(list2.get(0).getPaymentDate(), DATE_FORMATTER);
        //     return date1.compareTo(date2);
        // });

        return groupedLists;
    }

    public static void main(String[] args) {
        // 模拟从数据库获取的支付列表
        List<Payment> payments = new ArrayList<>();
        payments.add(new Payment(1, "2023-01-15", 100));
        payments.add(new Payment(2, "2023-01-16", 200));
        payments.add(new Payment(3, "2023-01-15", 150));
        payments.add(new Payment(4, "2023-01-17", 300));
        payments.add(new Payment(5, "2023-01-16", 250));
        payments.add(new Payment(6, "2023-01-15", 50));
        payments.add(new Payment(7, "2023-01-17", 120));
        // 添加一个日期格式错误的记录以测试异常处理
        payments.add(new Payment(8, "2023/01/18", 90)); // 格式错误

        PaymentGrouper grouper = new PaymentGrouper();
        List<List<Payment>> groupedPayments = grouper.groupPaymentsByDate(payments);

        // 打印分组结果
        System.out.println("Grouped Payments:");
        for (List<Payment> dailyPayments : groupedPayments) {
            System.out.println("--- Date: " + dailyPayments.get(0).getPaymentDate().substring(0, 10) + " ---"); // 假设同一天
            for (Payment p : dailyPayments) {
                System.out.println(p);
            }
        }
    }
}