What does the Stream count() method do, and how does it interact with filter()?

count() is a terminal operation that returns the number of elements in the stream as a long. Chained after filter(), e.g. list.stream().filter(p).count(), it's an efficient way to count how many elements satisfy a condition — Java's Stream implementation can even skip actually materializing elements in some cases, computing the count more efficiently than a full iteration when the pipeline allows it.