与流行为不同的平行流

Question

    List<String> list = Arrays.asList("1", "2", "3");
    String resultParallel = list.parallelStream().collect(StringBuilder::new,
            (response, element) -> response.append(" ").append(element),
            (response1, response2) -> response1.append(",").append(response2.toString()))
            .toString();
    System.out.println("ResultParallel: " + resultParallel);

    String result = list.stream().collect(StringBuilder::new,
            (response, element) -> response.append(" ").append(element),
            (response1, response2) -> response1.append(",").append(response2.toString()))
            .toString();

    System.out.println("Result: " + result);

结果：1 2 3

有人能解释为什么会发生这种情况，以及我如何让非并行版本给出与并行版本相同的结果吗？

Answer 1

另请注意，即使在组合器中用空格替换、，结果仍然会有所不同（稍微修改了代码，使其更具可读性）：

String resultParallel = list.parallelStream().collect(
            StringBuilder::new,
            (builder, elem) -> builder.append(" ").append(elem),
            (left, right) -> left.append(" ").append(right)).toString();

    String result = list.stream().collect(
            StringBuilder::new,
            (builder, elem) -> builder.append(" ").append(elem),
            (left, right) -> left.append(" ").append(right)).toString();


  System.out.println("ResultParallel: ->" + resultParallel + "<-"); // -> 1  2  3  4<-
  System.out.println("Result: ->" + result + "<-"); // -> 1 2 3 4<-

注意你的空间太多了。

java-doc有以下提示：

String.join(",", yourList)
yourList.stream().collect(Collectors.joining(","))