/** The key of the message: File System Config Name */
    public static final String LABELS_FILE_CONFIG_NAME = "{labels.fileConfigName}";

    /** The key of the message: File name */
    public static final String LABELS_FILE_NAME = "{labels.fileName}";

    /** The key of the message: Handler Name */
    public static final String LABELS_HANDLER_NAME = "{labels.handlerName}";

   *                 return countDifferentLines(file1Reader, file2Reader);
   *               },
   *               executor)
   *           .closing(executor);
   * }</pre>
   */
  // TODO(cpovirk): Use simple name instead of fully qualified after we stop building with JDK 8.
  @com.google.errorprone.annotations.DoNotMock(
      "Use ClosingFuture.whenAllSucceed() or .whenAllComplete() instead.")
  public static class Combiner {

    return new TreeMap<>();
  }

  /**
   * Creates a <i>mutable</i> {@code TreeMap} instance with the same mappings as the specified map
   * and using the same ordering as the specified map.
   *
   * <p><b>Note:</b> if mutability is not required, use {@link
   * ImmutableSortedMap#copyOfSorted(SortedMap)} instead.
   *