code: private File getCollectionDir(String projectId, String eventCollection) throws IOException {
          File collectionDir = new File(getProjectDir(projectId, true), eventCollection);
          if (!collectionDir.exists()) {
              KeenLogging.log("Cache directory for event collection '" + eventCollection +
                      "' doesn't exist. Creating it.");
              if (!collectionDir.mkdirs()) {
                  throw new IOException("Could not create collection cache directory '" +
                          collectionDir.getAbsolutePath() + "'");
              }
          }
          return collectionDir;
      }
docs: Gets the directory for events in the given collection. Creates the directory (and any
  necessary parents) if it does not exist already.
  
  @param projectId       The project ID.
  @param eventCollection The name of the event collection.
  @return The directory for events in the collection.