* removal operations, and these are reflected in the underlying map.
   *
   * <p>It's acceptable for the underlying map to contain null keys, and even null values provided
   * that the function is capable of accepting null input. The transformed map might contain null
   * values, if the function sometimes gives a null result.
   *
   * <p>The returned map is not thread-safe or serializable, even if the underlying map is.

   * removal operations, and these are reflected in the underlying map.
   *
   * <p>It's acceptable for the underlying map to contain null keys, and even null values provided
   * that the function is capable of accepting null input. The transformed map might contain null
   * values, if the function sometimes gives a null result.
   *
   * <p>The returned map is not thread-safe or serializable, even if the underlying map is.