Why it matters
Streams make Java code shorter and clearer for typical data-processing tasks. What used to be a 10-line loop becomes a 2-line pipeline. Modern Java code uses streams extensively.
The architecture
Building a stream: collections have .stream() methods. Arrays via Arrays.stream(). Files via Files.lines(). Generator methods produce infinite streams.
Intermediate operations: filter, map, flatMap, sorted, distinct, limit. Each returns a new stream.
Terminal operations: forEach, collect, reduce, count, findFirst. Triggers pipeline execution.
How it works end to end
Collectors: toList, toSet, toMap, groupingBy, joining. These aggregate stream contents into standard results.
Parallel streams: .parallelStream() distributes work across a ForkJoinPool. Careful — overhead makes them slower for small streams; contention limits scalability.
Optional: methods like findFirst return Optional<T> to represent absence explicitly, avoiding null.