side effect mapUsingReduce

MapApplication.java

public class MapApplication {

    // case 1
    public static <I, O> List<O> mapUsingCollect(Stream<I> stream, Function<I, O> mapper) {
        return stream.collect(ArrayList::new, (acc, element) -> acc.add(mapper.apply(element)), ArrayList::addAll);
    }

    // case 2
    public static <I, O> List<O> mapUsingReduce(Stream<I> stream, Function<I, O> mapper) {
        return stream.reduce(new ArrayList<>(), (acc, element) -> {
            ArrayList<O> newAcc = new ArrayList<>(acc);

            newAcc.add(mapper.apply(element));

            return newAcc;
        }, (leftPartialResult, rightPartialResult) -> {
            ArrayList<O> newLeftPartialResult = new ArrayList<>(leftPartialResult);
            newLeftPartialResult.addAll(rightPartialResult);

            return newLeftPartialResult;
        });
    }

    // case 3
    public static <I, O> List<O> mapUsingReduce2(Stream<I> stream, Function<I, O> mapper) {
        return stream.reduce(new ArrayList<>(), (acc, element) -> {
            acc.add(mapper.apply(element));

            return acc;
        }, (leftPartialResult, rightPartialResult) -> leftPartialResult);
    }

    public static void main(String[] args) {
        System.out.println(mapUsingCollect(Stream.of(1, 2, 3, 4), x -> x + 1)); //[2, 3, 4, 5]
        System.out.println(mapUsingCollect(Stream.of(1, 2, 3, 4).parallel(), x -> x + 1)); //[2, 3, 4, 5]
        
        System.out.println(mapUsingReduce(Stream.of(1, 2, 3, 4), x -> x + 1)); //[2, 3, 4, 5]
        System.out.println(mapUsingReduce(Stream.of(1, 2, 3, 4).parallel(), x -> x + 1)); //[2, 3, 4, 5]

        System.out.println(mapUsingReduce2(Stream.of(1, 2, 3, 4), x -> x + 1)); //[2, 3, 4, 5]
        System.out.println(mapUsingReduce2(Stream.of(1, 2, 3, 4).parallel(), x -> x + 1)); //[null, 3, 2, 4]
    }
}