Java Stream API - Data Processing

1public class DataProcessor {
2    public Map<String, List<Transaction>> groupByCategory(
3            List<Transaction> transactions) {
4        return transactions.stream()
5            .filter(t -> t.getAmount() > 0)
6            .sorted(Comparator.comparing(Transaction::getDate).reversed())
7            .collect(Collectors.groupingBy(Transaction::getCategory));
8    }
9 
10    public Optional<Transaction> findHighestTransaction(
11            List<Transaction> transactions, String category) {
12        return transactions.stream()
13            .filter(t -> t.getCategory().equals(category))
14            .max(Comparator.comparing(Transaction::getAmount));
15    }
16 
17    public double calculateAverage(List<Transaction> transactions) {
18        return transactions.stream()
19            .mapToDouble(Transaction::getAmount)
20            .average()
21            .orElse(0.0);
22    }
23}