What is a Stream?

A Stream is a pipeline of operations on a sequence of elements. It does not store data — it computes on demand. Streams are great for expressing "do X to every element, keep the ones that match Y, then reduce to one value" without writing explicit loops.

Three kinds of operations

1. Source — list.stream(), Arrays.stream(arr), Stream.of(...).
2. Intermediate — return another stream. Lazy. filter, map, flatMap, sorted, distinct, limit, skip.
3. Terminal — trigger the pipeline. collect, forEach, count, reduce, anyMatch, findFirst.

Common Recipes

Filter + Map + Collect

List<String> names = people.stream()
    .filter(p -> p.age >= 18)
    .map(p -> p.name.toUpperCase())
    .collect(Collectors.toList());

Sum / Reduce

int total = nums.stream().mapToInt(Integer::intValue).sum();
int product = nums.stream().reduce(1, (a, b) -> a * b);

Grouping

Map<String, List<Student>> byBranch =
    students.stream().collect(Collectors.groupingBy(s -> s.branch));

Rules of Thumb

• Streams are single-use — once a terminal op runs, you can't reuse it. • Prefer streams when the data flow is clear; stick with for loops for stateful logic with side effects. • Use parallelStream() only after measuring — it isn't always faster.