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Streams Beginner

Optional.ifPresentOrElse()

Handle both present and empty cases of Optional in one call.

Optional

Code Comparison

✕ Java 8

Optional<User> user = findUser(id);
if (user.isPresent()) {
    greet(user.get());
} else {
    handleMissing();
}

✓ Java 9+

findUser(id).ifPresentOrElse(
    this::greet,
    this::handleMissing
);

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Why the modern way wins

📏

Single expression

Both cases handled in one method call.

🚫

No get()

Eliminates the dangerous isPresent() + get() pattern.

🔗

Fluent

Chains naturally after findUser() or any Optional-returning method.

Old Approach

if/else on Optional

Modern Approach

ifPresentOrElse()

Since JDK

Difficulty

Beginner

JDK Support

Optional.ifPresentOrElse()

Available

Widely available since JDK 9 (Sept 2017)

How it works

ifPresentOrElse() takes a Consumer for the present case and a Runnable for the empty case. It avoids the isPresent/get anti-pattern.

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 Advanced

Stream gatherers

Java 8

// Sliding window: manual implementation
List<List<T>> windows = new ArrayList<>();
for (int i = 0; i <= list.size()-3; i++) {
    windows.add(
        list.subList(i, i + 3));
}

Java 24+

var windows = stream
    .gather(
        Gatherers.windowSliding(3)
    )
    .toList();

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JDK 24+ →

Streams Intermediate

Collectors.flatMapping()

Java 8

// Flatten within a grouping collector
// Required complex custom collector
Map<String, Set<String>> tagsByDept =
    // no clean way in Java 8

Java 9+

var tagsByDept = employees.stream()
    .collect(groupingBy(
        Emp::dept,
        flatMapping(
            e -> e.tags().stream(),
            toSet()
        )
    ));

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JDK 9+ →