Example of sequence and traverse for java's CompletableFuture.

FutureOps.java

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.function.Function;

public class FutureOps {

  public static  <T> CompletableFuture<List<T>> sequence(List<CompletableFuture<T>> ls) {
    return ls.stream()
      .reduce(
        CompletableFuture.completedFuture(new ArrayList<T>(ls.size())),
        (fs, fa) -> fs.thenCombine(fa, (s, a) -> {s.add(a); return s;}),
        (fa, fb) -> fa.thenCombine(fb, (a, b) -> {a.addAll(b); return a;})
      ).thenApply(List::copyOf);
  }
  
  public static  <T, V> CompletableFuture<List<V>> traverse(List<T> ls, Function<T, CompletableFuture<V>> fn) {
    return ls.stream()
      .reduce(
        CompletableFuture.completedFuture(new ArrayList<V>(ls.size())),
        (fs, a) -> fs.thenCombine(fn.apply(a), (s, f) -> {s.add(f); return s;}),
        (fa, fb) -> fa.thenCombine(fb, (a, b) -> {a.addAll(b); return a;})
      ).thenApply(List::copyOf);
  }
}

Thanks, very helpful :)