public CompletableFuture<Boolean> doA();
public CompletableFuture<Boolean> doB();

return doA() || doB();

doA.thenApply(b -> {
  if (!b) {
    return doB();
  } else {
    return CompletableFuture.completedFuture(b);
  }
}

CompletableFuture<Boolean> result = doA().thenCompose(b -> b
    ? CompletableFuture.completedFuture(Boolean.TRUE) : doB());

static <T> CompletableFuture<T> flatten(
  CompletableFuture<CompletableFuture<T>> nestedFuture) {
    return nestedFuture.thenCompose(Function.identity());
}