我希望我正确地理解了你的意思，所以这里是：

CompletableFuture<List<IDS1>> ids1Future = CompletableFuture.supplyAsync(service::find);

   CompletableFuture<Map<IDS1, List<IDS2>>> map = ids1Future.thenApply(listIDS1 -> {
        List<Entry<IDS1, CompletableFuture<List<IDS2>>>> list =
            listIDS1
                .stream()
                .map(ids1 -> new SimpleEntry<>(ids1, CompletableFuture.supplyAsync(() -> service2.getById(ids1))))
                .collect(Collectors.toList());

        return list.stream()
                   .map(entry -> new SimpleEntry<>(entry.getKey(), entry.getValue().join()))
                   .collect(Collectors.toMap(Entry::getKey, Entry::getValue));

    });

    Map<IDS1, List<IDS2>> result = map.join();

这是一个如何使用fork/join将字符列表转换为字符串列表的示例，其中每个字符串是字符的n倍的复制。你可以根据自己的情况调整这个例子。
我希望这就是你要找的。

import java.util.Collection;
import java.util.List;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.ForkJoinTask;
import java.util.concurrent.RecursiveTask;
import java.util.stream.Collectors;

public class ForkJoinExample {

    public static class RepeatTask extends RecursiveTask<List<String>> {

        private final List<Character> chars;
        private final int repeatCount;

        public RepeatTask(List<Character> chars, int repeatCount) {
            this.chars = chars;
            this.repeatCount = repeatCount;
        }

        public RepeatTask(Character c, int repeatCount) {
            this.chars = List.of(c);
            this.repeatCount = repeatCount;
        }

        @Override
        protected List<String> compute() {
            if (chars.size() > 1) {
                ForkJoinTask.invokeAll(chars.stream().map(c -> new RepeatTask(c, repeatCount)).collect(Collectors.toList()));
                return ForkJoinTask.invokeAll(chars.stream().map(c -> new RepeatTask(c, repeatCount)).collect(Collectors.toList()))
                        .stream()
                        .map(ForkJoinTask::join)
                        .flatMap(Collection::stream)
                        .collect(Collectors.toList());
            }
            return List.of(String.valueOf(chars.get(0)).repeat(Math.max(0, repeatCount)));
        }

    }

    public static void main(String[] args) {
        ForkJoinPool commonPool = ForkJoinPool.commonPool();
        RepeatTask task = new RepeatTask(List.of('a', 'b', 'c'), 4);
        commonPool.execute(task);
        System.out.println(task.join());
    }
}

结果是： [aaaa, bbbb, cccc]