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Streams advanced

Stream gatherers

Use gatherers for custom intermediate stream operations.

Code Comparison
✕ 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();
Why the modern way wins
🧩

Composable

Gatherers compose with other stream operations.

📦

Built-in operations

windowFixed, windowSliding, fold, scan out of the box.

🔧

Extensible

Write custom gatherers for any intermediate transformation.

Old Approach
Custom Collector
Modern Approach
gather()
Since JDK
24
Difficulty
advanced
JDK Support
Stream gatherers
Available

Finalized in JDK 24 (JEP 485, March 2025).

How it works

Gatherers are a new intermediate stream operation that can express complex transformations like sliding windows, fixed-size groups, and scan operations that were impossible with standard stream ops.

Related patterns

Streams beginner

Stream.ofNullable()

Java 8
Stream<String> s = val != null
    ? Stream.of(val)
    : Stream.empty();
Java 9+
Stream<String> s =
    Stream.ofNullable(val);
Hover to see modern ➜
JDK 9+ →
Streams intermediate

Stream.iterate() with predicate

Java 8
Stream.iterate(1, n -> n * 2)
    .limit(10)
    .forEach(System.out::println);
// can't stop at a condition
Java 9+
Stream.iterate(
    1,
    n -> n < 1000,
    n -> n * 2
).forEach(System.out::println);
// stops when n >= 1000
Hover to see modern ➜
JDK 9+ →
Streams beginner

Stream takeWhile / dropWhile

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)
Hover to see modern ➜
JDK 9+ →
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