Assuming I have a Java IntStream, is it possible to convert it to an IntStream with cumulative sums? For example, a stream starting with [4, 2, 6, …] should be converted to [4, 6, 12, …].
More generally, how should one go about implementing stateful stream operations? It feels like this should be possible:
myIntStream.map(new Function<Integer, Integer> {
int sum = 0;
Integer apply(Integer value){
return sum += value;
}
);
With the obvious restriction that this works only on sequential streams. However, Stream.map explicitely requires a stateless map function. Am I right in missing a Stream.statefulMap or Stream.cumulative operation or is that missing the point of Java streams?
Compare for example to Haskell, where the scanl1 function solves exactly this example:
scanl1 (+) [1 2 3 4] = [1 3 6 10]
Answer
You can do this with an atomic number. For example:
import java.util.concurrent.atomic.AtomicLong;
import java.util.stream.IntStream;
import java.util.stream.LongStream;
public class Accumulator {
public static LongStream toCumulativeSumStream(IntStream ints){
AtomicLong sum = new AtomicLong(0);
return ints.sequential().mapToLong(sum::addAndGet);
}
public static void main(String[] args){
LongStream sums = Accumulator.toCumulativeSumStream(IntStream.range(1, 5));
sums.forEachOrdered(System.out::println);
}
}
This outputs:
1 3 6 10
I’ve used a Long to store the sums, because it’s entirely possible that two ints add up to well over Integer.MAX_VALUE, and a long has less of a chance of overflow.
