flatMap in Depth The Collectors API covers perhaps 90% of real-world aggregation needs, but the moment you need something beyond grouping, partitioning, or joining, the custom Collector is there. Understanding flatMap and the full Collectors toolkit — including when to write your own — separates competent Java 8 developers from those who still reach for a for loop whenever a problem gets slightly unusual. flatMap is the most powerful transformation in the Streams API.