Stream APIとは何ですか?

// ❌ imperative — explicit loop, mutable accumulator
List<String> result = new ArrayList<>();
for (Person p : people) {
    if (p.getAge() >= 18) {
        result.add(p.getName().toUpperCase());
    }
}

// ✅ stream — declarative pipeline, reads like a description of the transformation
List<String> result = people.stream()
    .filter(p -> p.getAge() >= 18)        // keep adults
    .map(p -> p.getName().toUpperCase())   // extract + transform names
    .collect(Collectors.toList());          // gather into a list

Source → Intermediate operations (lazy) → Terminal operation (triggers execution)

stream()                    // source
  .filter(...)  .map(...)   // intermediate — lazy, return a new stream, chainable
  .collect(...)             // terminal — produces a result, RUNS the pipeline

list.stream()
    .filter(x -> x > 0)              // keep matching
    .map(x -> x * 2)                 // transform each
    .sorted()                        // sort
    .distinct()                      // remove duplicates
    .limit(10)                       // take first 10
    .collect(Collectors.toList());

// aggregations
int sum = nums.stream().mapToInt(Integer::intValue).sum();
long count = list.stream().filter(...).count();
Optional<Person> first = people.stream().filter(...).findFirst();
boolean any = list.stream().anyMatch(x -> x > 100);

// grouping (very powerful)
Map<String, List<Person>> byCity =
    people.stream().collect(Collectors.groupingBy(Person::getCity));

int total = nums.stream().reduce(0, (a, b) -> a + b);   // sum via reduction

list.parallelStream().filter(...).collect(...);   // process across multiple cores
// ⚠️ use only for large data + independent/stateless operations; measure first