@Nullable
  private String getAdjacentContentEntry(final String contentEntryUrl, int delta) {
    final ContentEntry[] contentEntries = getModel().getContentEntries();
    for (int idx = 0; idx < contentEntries.length; idx++) {
      ContentEntry entry = contentEntries[idx];
      if (contentEntryUrl.equals(entry.getUrl())) {
        int nextEntryIndex = (idx + delta) % contentEntries.length;
        if (nextEntryIndex < 0) {
          nextEntryIndex += contentEntries.length;
        }
        return nextEntryIndex == idx ? null : contentEntries[nextEntryIndex].getUrl();
      }
    }
    return null;
  }
@Nullable
  private String getAdjacentContentEntry(final String contentEntryUrl, int delta)