• 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(@Nullable Collection<T> collection) {
    return Optional.ofNullable(collection)
        .map(Collection::stream)
        .orElse(Stream.empty());
}
 
public static <T> Stream<T> asStream(@Nullable T[] array) {
    return Optional.ofNullable(array)
        .map(Arrays::stream)
        .orElse(Stream.empty());
}