Processing data with the Stream API

The Stream API, added in Java SE 8, implements the map-filter-reduce algorithm as a companion to the Collections Framework, which stores and organizes data in JVM memory. Open a stream on a collection, or create streams on I/O or custom data sources. Mapping transforms objects one-to-one via a Function, filtering selects elements via a Predicate, and reducing aggregates like SQL's COUNT, SUM, MIN, MAX, or AVERAGE. Intermediate operations return streams and process nothing until a terminal operation consumes elements one at a time; a stream stores no data. Call only one method per stream instance — reuse throws IllegalStateException — so create and consume streams on the spot, never modify the source, and avoid side effects. Scope: Java. Reviewed October 4, 2026.