Convert Java java.util.Stream<Byte> 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();
}

Answer 1

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.