首先，没有必要组合器在非并行流中执行，因为没有什么可以组合。

First off, there is no need for the combiner to be executed in a non-parallel stream since there is nothing to combine.

其次，你的问题源于使用 String :: new 和 String.concat 。累加器应该通过将第二个参数与它组合来修改第一个参数，但由于Java中的字符串是不可变的，因此代码将不会产生任何内容。

Secondly, your issue stems from using String::new and String.concat. The accumulator is supposed to modify the first argument by combining the second argument with it but since strings in Java are immutable your code will produce nothing.

          res=res.concat(" ").concat(elem);

将创建一个新字符串然后扔掉它。您希望使用StringBuilder，以便保留中间结果：

will create a new string and then throw away it. You want to use a StringBuilder instead so you can keep the intermediate results:

public static void main(String[] args) {
    List<String> list2 = Arrays.asList("adf", "bcd", "abc", "hgr", "jyt", "edr", "biu");

    String collect = list2.stream().collect(StringBuilder::new, (res, elem) -> {
        res.append(" ").append(elem);
    }, (res1, res2) -> {
        res1.append(res2.toString());
        System.out.printf("res1=%s, res2=%s\n", res1, res2);
    }).toString();
    System.out.println("collect=" + collect);
}

这对于并行流也能正常工作

This will also work correctly with a parallel stream

res1 = hgr jyt，res2 = jyt

res1 = bcd abc，res2 = abc

res1 = adf bcd abc ，res2 = bcd abc

res1 = edr biu，res2 = biu

res1 = hgr jyt edr biu，res2 = edr biu

res1 = adf bcd abc hgr jyt edr biu，res2 = hgr jyt edr biu

collect = adf bcd abc hgr jyt edr biu

res1= hgr jyt, res2= jyt
res1= bcd abc, res2= abc
res1= adf bcd abc, res2= bcd abc
res1= edr biu, res2= biu
res1= hgr jyt edr biu, res2= edr biu
res1= adf bcd abc hgr jyt edr biu, res2= hgr jyt edr biu
collect= adf bcd abc hgr jyt edr biu