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Streams Beginner

Stream takeWhile / dropWhile

Take or drop elements from a stream based on a predicate.

Code Comparison

✕ Java 8

List<Integer> result = new ArrayList<>();
for (int n : sorted) {
    if (n >= 100) break;
    result.add(n);
}
// no stream equivalent in Java 8

✓ Java 9+

var result = sorted.stream()
    .takeWhile(n -> n < 100)
    .toList();
// or: .dropWhile(n -> n < 10)

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Why the modern way wins

🎯

Short-circuit

Stops processing as soon as the predicate fails.

🔗

Pipeline-friendly

Chain with other stream operations naturally.

📖

Declarative

takeWhile reads like English: 'take while less than 100'.

Old Approach

Manual Loop

Modern Approach

takeWhile/dropWhile

Since JDK

Difficulty

Beginner

JDK Support

Stream takeWhile / dropWhile

Available

Widely available since JDK 9 (Sept 2017)

How it works

takeWhile() returns elements while the predicate is true and stops at the first false. dropWhile() skips elements while true and returns the rest. Both work best on ordered streams.

Stream.toList()

Java 8

List<String> result = stream
    .filter(s -> s.length() > 3)
    .collect(Collectors.toList());

Java 16+

List<String> result = stream
    .filter(s -> s.length() > 3)
    .toList();

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JDK 16+ →

Streams Intermediate

Stream.mapMulti()

Java 8

stream.flatMap(order ->
    order.items().stream()
        .map(item -> new OrderItem(
            order.id(), item)
        )
);

Java 16+

stream.<OrderItem>mapMulti(
    (order, downstream) -> {
        for (var item : order.items())
            downstream.accept(
                new OrderItem(order.id(), item));
    }
);

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JDK 16+ →

Streams Intermediate

Optional.or() fallback

Java 8

Optional<Config> cfg = primary();
if (!cfg.isPresent()) {
    cfg = secondary();
}
if (!cfg.isPresent()) {
    cfg = defaults();
}

Java 9+

Optional<Config> cfg = primary()
    .or(this::secondary)
    .or(this::defaults);

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JDK 9+ →