Victor Stafusa Victor Stafusa - 9 days ago 5
Java Question

Understanding Spliterator, Collector and Stream in Java 8

I am having trouble understanding the

interface in Java 8, especially where it has to do with the
Spliterator
and
Collector
interfaces. My problem is that I simply can't understand yet the
Spliterator
and the
Collector
interfaces, and as a result the
Stream
interface is still somewhat obscure to me.

What exactly is a
Spliterator
and a
Collector
, and how can I use them? If I am willing to write my own
Spliterator
or
Collector
(and probably my own
Stream
in that process), what should I do and not do?

I read some examples scattered around the web, but since everything here is stil new and subject to changes, examples and tutorials are still very sparse.

Answer

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 include:

  • summing, e.g. Collectors.reducing(0, (x, y) -> x + y)
  • StringBuilder appending, e.g. Collector.of(StringBuilder::new, StringBuilder::append, StringBuilder::append, StringBuilder::toString)
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