Here my related code:

  public String build() {
    StringBuilder s3Key = new StringBuilder();

    if (bucketName == null || fileName == null) {
      throw new IllegalStateException("Bucket name and file name are required");
    }

    s3Key.append(bucketName);

    String prefix = paths.stream().filter(Predicate.not(Strings::isNullOrEmpty)).collect(Collectors.joining("/"));
    s3Key.append(prefix);

    s3Key.append(fileName);

    return s3Key.toString();
  }

Is there any way to collect streamed string into existent s3Key StringBuilder?