Can Java’s Stream.collect() return null?

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.

From JavaDoc of Collector:

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.