如何使用Java 8 streams将数据按月和年分组

import java.time.LocalDate;
import java.util.ArrayList;
import java.util.List;

public class Transaction {
    private LocalDate date;
    private double amount;

    public Transaction(LocalDate date, double amount) {
        this.date = date;
        this.amount = amount;
    }

    public LocalDate getDate() {
        return date;
    }

    public double getAmount() {
        return amount;
    }
}

public class Main {
    public static void main(String[] args) {
        List<Transaction> transactions = new ArrayList<>();
        transactions.add(new Transaction(LocalDate.of(2022, 1, 1), 100.0));
        transactions.add(new Transaction(LocalDate.of(2022, 1, 15), 200.0));
        transactions.add(new Transaction(LocalDate.of(2022, 2, 1), 150.0));
        transactions.add(new Transaction(LocalDate.of(2022, 2, 15), 250.0));
        transactions.add(new Transaction(LocalDate.of(2022, 3, 1), 300.0));
        transactions.add(new Transaction(LocalDate.of(2022, 3, 15), 350.0));
    }
}

import java.time.format.DateTimeFormatter;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        List<Transaction> transactions = new ArrayList<>();
        transactions.add(new Transaction(LocalDate.of(2022, 1, 1), 100.0));
        transactions.add(new Transaction(LocalDate.of(2022, 1, 15), 200.0));
        transactions.add(new Transaction(LocalDate.of(2022, 2, 1), 150.0));
        transactions.add(new Transaction(LocalDate.of(2022, 2, 15), 250.0));
        transactions.add(new Transaction(LocalDate.of(2022, 3, 1), 300.0));
        transactions.add(new Transaction(LocalDate.of(2022, 3, 15), 350.0));

        // 按月分组
        Map<String, List<Transaction>> transactionsByMonth = transactions.stream()
                .collect(Collectors.groupingBy(transaction -> transaction.getDate().format(DateTimeFormatter.ofPattern("yyyy-MM"))));

        // 按年分组
        Map<String, List<Transaction>> transactionsByYear = transactions.stream()
                .collect(Collectors.groupingBy(transaction -> transaction.getDate().format(DateTimeFormatter.ofPattern("yyyy"))));

        // 打印按月分组结果
        transactionsByMonth.forEach((key, value) -> {
            System.out.println("Month: " + key);
            value.forEach(transaction -> System.out.println("Transaction: " + transaction.getAmount()));
        });

        // 打印按年分组结果
        transactionsByYear.forEach((key, value) -> {
            System.out.println("Year: " + key);
            value.forEach(transaction -> System.out.println("Transaction: " + transaction.getAmount()));
        });
    }
}