Convert Java java.util.Stream to a byte array

Question

I need to convert a stream (Java 8 stream, not InputStream) into a byte array.

I have tried using toArray method of Stream, but it doesn't accept byte[]::new, as byte is a primitive type.

I also tried using toArray method of IntStream, but it doesn't accept arguments, it assumes you want to get int[] back.

It's possible to convert a stream to a list or an array of boxed Byte types, and then convert it to byte[], but I wonder if there is a better way.

Edit: Added code, if that helps understanding the question. This doesn't compile, it's just pseudo-code. If it helps, you can also consider IntStream to be Stream<Byte>, for my purposes it doesn't matter.

byte[] getArray(IntStream stream) {
    return stream.toArray();
}

How about writing the bytes into a `ByteArrayOutputStream` and returning that from the stream? — Henrik Aasted Sørensen, Feb 23 '18 at 09:53
show us some code? Will help to get an idea about what you want to try. — ProgrammerBoy, Feb 23 '18 at 09:56
Related: https://stackoverflow.com/questions/32459683/in-java-8-is-there-a-bytestream-class — Andy Turner, Feb 23 '18 at 09:59
Why do you think you can convert the stream to a byte array? It would require serialization at the minimum. — Kayaman, Feb 23 '18 at 10:03
@daniu `int` or `Integer`, but it doesn't really matter, whatever works for the solution (question of using `map` vs `mapToObj`). — Konrad Borowski, Feb 23 '18 at 10:46
@Kayaman Because Java provides `toArray` methods for `IntStream`, `LongStream`, `DoubleStream` and `Stream`. It seems like there should be a way to convert to `byte[]`. — Konrad Borowski, Feb 23 '18 at 10:54
Well it might seem like that to someone with no understanding about Java. You can't just convert an object to `byte[]`, let alone a stream of objects. What do you think you'd do with those bytes? — Kayaman, Feb 23 '18 at 10:56
@Kayaman I know that they aren't represented the same way, so copying is needed, but that's fine. What I have is `Stream` (or `IntStream`). — Konrad Borowski, Feb 23 '18 at 10:57
Why do you have a `Stream` and not an `InputStream`? Did someone use `Stream` because they didn't really understand Java 8 streams? Because that's the only reason I could think of. — Kayaman, Feb 23 '18 at 11:00
@Kayaman Because it comes from a stream over a collection which did some other transformations on a value. — Konrad Borowski, Feb 23 '18 at 11:02

daniu · Answer 1 · 2018-02-23T10:52:17.977

I guess you can make byte[] from the original stream's individual entries and then concatenate them in the end.

static byte[] <T> toByteArray(Stream<T> stream, Function<T, byte[]> transform) {
    Stream<byte[]> arrayStream = stream.map(transform);
    arrayStream.collect(concatArrays);
}

However, the only implementation I can think of for a concatArrays Collector would be:

maintain a byte[] containing the already collected data
for each entry of the Stream, expand it to accomodate the new entry
use System.arraycopy to combine the arrays

I'd expect you'd be better off with your "boxing to Byte and then convert" in almost all cases.

Convert Java java.util.Stream to a byte array

1 Answers1