The Stream Pipeline

A stream is a conveyor belt for data: you line up processing steps, and elements flow through them one stage at a time.

List<String> names = List.of("Ada", "bob", "Grace", "eve", "Turing");

List<String> result = names.stream()          // 1. source
    .filter(n -> n.length() > 3)              // 2. intermediate: keep some
    .map(String::toUpperCase)                 // 3. intermediate: transform
    .sorted()                                 // 4. intermediate: order
    .collect(java.util.stream.Collectors.toList());  // 5. terminal: produce
// [ADA, GRACE, TURING]

The vocabulary:

• Source - list.stream(), Stream.of(...), map.entrySet().stream().
• Intermediate operations (return a new stream, are lazy - nothing runs until a terminal op exists): filter, map, sorted, distinct, limit, skip, peek.
• Terminal operations (actually run the belt, once): collect, forEach, count, anyMatch/allMatch/noneMatch, findFirst, reduce.

Primitive streams avoid boxing

int total = prices.stream()            // Stream<Double>
    .mapToInt(Double::intValue)        // IntStream - no Double objects
    .sum();

mapToInt/mapToLong/mapToDouble switch to primitive streams with sum(), average(), max(), count() ready-made.

Collectors you will actually use

Collectors.toList()                              // the everyday one
Collectors.joining(", ")                         // "Ada, Grace"
Collectors.groupingBy(String::length)            // Map<Integer, List<String>>
Collectors.counting()                            // usually inside groupingBy

The honesty section: when a loop is clearer

Streams are not a badge of honor. Prefer a plain loop when:

• you're accumulating with tricky state (running indexes, two counters stepping on each other);
• you need early exit with side effects mid-computation;
• the chain would run five stages deep and nobody can read it;
• you're processing two collections in lockstep.

A good rule: use streams for describe-a-transformation code ("filter the valid, map to names, collect"), loops for drive-a-procedure code. If a teammate must squint at your stream, write the loop.