• Java performance tutorial – How fast are the Java 8 streams?
• Java SE 8 new features tour: Processing Collections with Streams API
• Stream Performance
• Java 8 Stream API

• Convert Iterable to Stream using Java 8 JDK

  StreamSupport.stream(iterable.spliterator(), false)
               .filter(...)
               .moreStreamOps(...);

• java - Turn a List of Lists into a List Using Lambdas - Stack Overflow

List<List<Object>> list = ...
List<Object> flat = 
    list.stream()
        .flatMap(List::stream)
        .collect(Collectors.toList());

public static <T> Stream<T> asStream(Collection<T> collection) {
    return Optional.ofNullable(collection)
        .map(Collection::stream)
        .orElse(Stream.empty());
}
 
public static <T> Stream<T> asStream(T[] array) {
    return Optional.ofNullable(array)
        .map(Arrays::stream)
        .orElse(Stream.empty());
}
 
public static <K, V> Stream<Map.Entry<K, V>> asStream(Map<K, V> map) {
    return Optional.ofNullable(map)
        .map(kvMap -> kvMap.entrySet().stream())
        .orElse(Stream.empty());
}

• 객체의 특정 field / property 를 통해 distinct를 하려면,
• 별도 Predicate 선언방식

  public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
      Map<Object,Boolean> seen = new ConcurrentHashMap<>(); // parallel stream에서 호출할까봐 이렇게 한듯.
      return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;
  }
   
  persons.stream().filter(distinctByKey(p -> p.getName());

• toMap이용

  // 특정 Key 필드로 Map을 만들되 중복 값은 모두 무시
  persons.stream()
    .collect(toMap(Person::getName, Function.identity(), (p1, p2) -> p1))
    .values();

• Java 8 Predicate Chain | Baeldung