You should almost certainly never have to deal with
Spliterator as a user; it should only be necessary if you're writing
Collection types yourself and also intending to optimize parallelized operations on them.
For what it's worth, a
Spliterator is a way of operating over the elements of a collection in a way that it's easy to split off part of the collection, e.g. because you're parallelizing and want one thread to work on one part of the collection, one thread to work on another part, etc.
You should essentially never be saving values of type
Stream to a variable, either.
Stream is sort of like an
Iterator, in that it's a one-time-use object that you'll almost always use in a fluent chain, as in the Javadoc example:
int sum = widgets.stream() .filter(w -> w.getColor() == RED) .mapToInt(w -> w.getWeight()) .sum();
Collector is the most generalized, abstract possible version of a "reduce" operation a la map/reduce; in particular, it needs to support parallelization and finalization steps. Examples of
Collectors.reducing(0, (x, y) -> x + y)
Collector.of(StringBuilder::new, StringBuilder::append, StringBuilder::append, StringBuilder::toString)