Collector.toList() will return an empty List for you.
Here is the implementation:
public static <T>
Collector<T, ?, List<T>> toList() {
return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
(left, right) -> { left.addAll(right); return left; },
CH_ID);
}
As you can see ArrayList::new is being used as a container for your items.
##Edit I see a lot of doubt here, so I read the JavaDoc and share with you:
From JavaDoc of Collector:
public interface Collector<T,A,R>
A mutable reduction operation that accumulates input elements into a mutable result container, optionally transforming the accumulated result into a final representation after all input elements have been processed. Reduction operations can be performed either sequentially or in parallel.
A Collector is specified by four functions that work together to accumulate entries into a mutable result container, and optionally perform a final transform on the result. They are:
- creation of a new result container (supplier())
- incorporating a new data element into a result container (accumulator())
- combining two result containers into one (combiner())
- performing an optional final transform on the container (finisher())
And
A sequential implementation of a reduction using a collector would create a single result container using the supplier function, and invoke the accumulator function once for each input element. A parallel implementation would partition the input, create a result container for each partition, accumulate the contents of each partition into a subresult for that partition, and then use the combiner function to merge the subresults into a combined result.
So as long as you don't do weird things like combine function return null, the Collector always return at least a mutable container using your provided supplier function.
And I think it's very counter-intuitive if an implementation would ever return null container.
