Advanced Streams & Collectors

Collectors supports grouping, joining, and building maps directly from a stream pipeline:

import java.util.stream.Collectors;

record Person(String name, String city) {}

List<Person> people = List.of(
    new Person("Ada", "London"),
    new Person("Grace", "New York"),
    new Person("Al", "London")
);

Map<String, List<Person>> byCity = people.stream()
    .collect(Collectors.groupingBy(Person::city));

String names = people.stream()
    .map(Person::name)
    .collect(Collectors.joining(", "));
// "Ada, Grace, Al"

A .parallelStream() splits work across multiple CPU cores automatically — useful for large, CPU-bound collections, but adds overhead that can make it slower for small ones.

Common mistakes

• Reaching for .parallelStream() by default — measure first; for small collections or I/O-bound work it typically doesn't help.