When are parallel streams beneficial in Java, and when can they actually hurt performance?

Parallel streams tend to help when the data set is large, roughly tens of thousands of elements or more, the work being done per element is CPU-bound rather than involving blocking I/O, the underlying data source splits cleanly (such as an ArrayList or a plain array), there is no shared mutable state being written to, and there's no requirement to preserve encounter order. They tend to hurt performance in several common situations: on small collections, where the overhead of splitting and coordinating threads outweighs any benefit; when the per-element work involves I/O, since that blocks threads in the shared ForkJoinPool.commonPool() and can starve every other parallel stream running in the same JVM; when the source is a LinkedList, which cannot be split efficiently; when a forEach writes into a shared mutable container, which reintroduces race conditions; and with certain stateful intermediate operations. As a rule, parallel streams should never be used for database queries or HTTP calls -- for that kind of work, CompletableFuture backed by a dedicated executor is the appropriate tool instead.