code: public static <R, C, V> Function<Table.Cell<R, C, V>, C> toColumnKeyFunction() {
      return new Function<Table.Cell<R, C, V>, C>() {
        @Override
        public C apply(final Table.Cell<R, C, V> input) {
          return input.getColumnKey();
        }
      };
    }
docs: Guava {@link Function} to transform a cell to its column key.