Streams: a parallel reduce needs an associative operation

long sequential = IntStream.rangeClosed(1, 1000).asLongStream().sum();
long parallel = IntStream.rangeClosed(1, 1000).parallel().asLongStream().sum();

System.out.println("sequential sum = " + sequential);
System.out.println("parallel   sum = " + parallel);
System.out.println("identical = " + (sequential == parallel));

String joined = Stream.of("a", "b", "c", "d").parallel()
        .collect(Collectors.joining());
System.out.println("parallel joining keeps order: " + joined);

sequential sum = 500500
parallel   sum = 500500
identical = true
parallel joining keeps order: abcd