GeeCON 2025: Ron Veen - Java 25: Unleashing the Power of Streams

Stream Gatherers in Java

Ron Veen opens at GeeCON by celebrating Java streams, which arrived with lambdas in Java 8 after a long wait, and introduces stream gatherers—previewed in Java 22–23 and finalized in Java 24—as the talk’s focus for Java 25-era development.

How Streams Execute

He revisits stream pipeline anatomy: sources, intermediate operations, and lazy terminal execution that pulls elements one at a time. He explains optimizations like fusion, short-circuiting, size-based splitting via spliterators, and the tradeoffs of parallel streams on small datasets.

Meet Gatherers

Gatherers extend the streams API with custom intermediate steps beyond built-in map and filter. Veen plans live coding a stateless alternative map, a stateful gatherer, and reimplementing a new default gatherer—grounded in understanding stateless versus stateful stream operations.

Why Internals Matter

Knowing how streams pull data and split work across threads helps developers write correct, efficient custom gatherers instead of treating streams as magic syntax.