Hello, in the case of non-concurrent streams, the processing order of the map method of the stream depends on the order in which the stream currently processes the upstream collection. The order of the ArrayList defaults to the natural order (insertion order), so the ArrayList collection is converted to the map method of the stream. Ability to process collection elements in natural order.

// 输出c b a
List<String> a = new LinkedList<String>();
a.add("c");
a.add("b");
a.add("a");
a.stream().map(x -> {
  System.out.println(x);
  return x;
}).count();

Map<String,String> m = new HashMap<String,String>();
m.put("4","D");
m.put("2","B");
m.put("1","A");
m.put("3","C");
// m.values() 默认顺序为A B C D
// 因此stream map的处理顺序也是 A B C D
m.values().stream().map(x -> {
                    System.out.println(x);
                    return x;
                }).count();

map Mapping, this has nothing to do with sorting.

List<String> list = new ArrayList<>();
list.add("A");
list.add("B");

list.stream().map((Function<String, Object>) s -> s + "1").forEach(System.out::println);
// output:
// A1
// B1

parallelStream does not guarantee the order, streamp guarantees the order