Stream API ve Tembellik

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public record CardTransaction(long id, String category, BigDecimal amount, TxStatus status) {
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    // category comes from the merchant category code: "Market", "Akaryakıt", "Restoran", …
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}

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public class LazinessDemo {
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    public static void main(String[] args) {
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        // The fraud rule wants the FIRST transaction above the review threshold.
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        Optional<CardTransaction> firstSuspicious = transactions().stream()
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                .peek(t -> System.out.println("filter  " + t.id()))
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                .filter(t -> t.amount().compareTo(new BigDecimal("5000")) > 0)
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                .peek(t -> System.out.println("map     " + t.id()))
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                .findFirst();
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        // filter  1
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        // filter  2
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        // map     2        <- transaction 2 is the first match: 3..1000 are never touched
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        System.out.println(firstSuspicious.map(CardTransaction::id).orElseThrow());   // 2
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        // Nothing at all is printed here: no terminal operation, so nothing runs.
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        transactions().stream().peek(t -> System.out.println("never " + t.id())).filter(t -> true);
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    }
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    static List<CardTransaction> transactions() {
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        return IntStream.rangeClosed(1, 1000)
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                .mapToObj(i -> new CardTransaction(i, i % 2 == 0 ? "Market" : "Akaryakıt",
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                        BigDecimal.valueOf(i == 2 ? 9_000 : 250), TxStatus.POSTED))
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                .toList();
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    }
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}

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public class InstallmentDates {
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    // A purchase in 6 installments: the next six monthly due dates.
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    // An infinite source is fine: limit() is short-circuiting, so only 6 are ever generated.
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    public static List<LocalDate> schedule(LocalDate firstDue, int installments) {
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        return Stream.iterate(firstDue, due -> due.plusMonths(1))
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                .map(InstallmentDates::nextBusinessDay)   // a due date never falls on a weekend
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                .limit(installments)
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                .toList();
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    }
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    private static LocalDate nextBusinessDay(LocalDate day) {
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        return switch (day.getDayOfWeek()) {
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            case SATURDAY -> day.plusDays(2);
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            case SUNDAY -> day.plusDays(1);
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            default -> day;
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        };
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    }
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}

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public class CategoryReport {
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    // The statement's "where did your money go" box: posted spending per category,
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    // highest first. Refunds and reversals are excluded. One pass, no intermediate lists.
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    public static List<Map.Entry<String, BigDecimal>> spendingByCategory(List<CardTransaction> txs) {
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        Map<String, BigDecimal> byCategory = txs.stream()
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                .filter(t -> t.status() == TxStatus.POSTED)
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                .collect(Collectors.groupingBy(CardTransaction::category,
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                        Collectors.reducing(BigDecimal.ZERO, CardTransaction::amount, BigDecimal::add)));
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        return byCategory.entrySet().stream()
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                .sorted(Map.Entry.<String, BigDecimal>comparingByValue().reversed())
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                .toList();
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    }
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}

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public class Pitfalls {
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    static void reuse(List<CardTransaction> txs) {
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        Stream<CardTransaction> posted = txs.stream().filter(t -> t.status() == TxStatus.POSTED);
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        long count = posted.count();
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        // posted.toList();   IllegalStateException: stream has already been operated upon or closed
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        // A stream is a one-shot pipeline. Keep a Supplier<Stream<CardTransaction>> for two passes.
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    }
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    static long countWithPeek(List<CardTransaction> txs, List<Long> seen) {
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        // Since Java 9, count() may compute the size of a sized source (a List, no filter)
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        // without running the pipeline: this peek never runs and `seen` stays empty.
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        // Never rely on peek for work; collect what you need instead.
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        return txs.stream().peek(t -> seen.add(t.id())).count();
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    }
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}