code: public DirectoryPollerBuilder setDefaultFileFilter(FileFilter filter) {
          if (filter == null) {
              throw new NullPointerException(NULL_ARGUMENT_ERROR_MESSAGE);
          }
          this.filter = filter;
          return this;
      }
docs: Set a {@link FileFilter} to be used. Only {@link FileElement}'s
  Satisfying the filter will be considered.
  <p>
  Optional setting. By default all {@link FileElement}'s are
  satisfying the filter.
  
  @param filter FileFilter
  @return {@link DirectoryPollerBuilder}
  @throws NullPointerException if <code>filter</code> is null.