[mperor]

What you'll learn

• Gatherers.windowSliding overlaps windows one element at a time, while Gatherers.windowFixed partitions the stream into non-overlapping chunks
• Gatherers.fold reduces a stream to a single final value without needing an associative/combiner function, unlike Stream.reduce
• Gatherers.scan emits every intermediate accumulated value instead of only the last one
• Gatherers.mapConcurrent maps elements concurrently (one virtual thread per element) but still delivers results in the original encounter order

Why it matters

These are the five built-in gatherers finalized by JEP 485 (Stream Gatherers). Knowing which one partitions, which one overlaps, which one streams intermediate state, and which one parallelizes safely while preserving order saves a trip to the Javadoc when reaching for a custom Collector/reduce alternative.

Read more

JEP 485: Stream Gatherers

Quiz

Given Stream.of(300, 100, 200, 50).gather(Gatherers.mapConcurrent(4, delay -> { sleep(delay); return delay; })), in what order do the results come out?

Answer

In input order: 300, 100, 200, 50 - even though the element with delay 50 finishes first. `mapConcurrent` runs the mapper for each element concurrently but buffers completed results so the output stream still respects encounter order.

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Note: this environment's network policy blocks access to the Foojay/Adoptium JDK download services the Gradle toolchain needs to provision JDK 25, and no JDK 25 is pre-installed, so :JavaReleases:test could not be run locally. Opening as a draft; CI (which has JDK 25) will verify. The gatherer APIs used (fold, scan, windowSliding, mapConcurrent) match the finalized JEP 485 signatures and mirror the style of the existing windowFixed/custom-gatherer tests already in the same file.

Closes #91

[github-actions]

Test Results

280 tests  +4   277 ✅ +4   6s ⏱️ ±0s
 80 suites ±0     3 💤 ±0 
 80 files   ±0     0 ❌ ±0 

Results for commit 51e8b79. ± Comparison against base commit ae4119f.

This pull request skips 1 and un-skips 1 tests.

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testLazyInitialized()

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testSerializationSafe()