Collectors turn streams into lists, sets, maps, grouped structures, and summaries.

Map<Integer, List<String>> byLength =
    names.stream()
        .collect(Collectors.groupingBy(String::length));

int totalLength =
    names.stream()
        .mapToInt(String::length)
        .sum();

groupingBy is useful for classification. partitioningBy splits values into two groups. Reduction operations combine elements into a result.

Prefer built-in collectors when they express the operation clearly. Custom collectors require careful supplier, accumulator, combiner, and finisher design.