code: public void put(String family, String alternate) {
          Args.nullNotPermitted(family, "family");
          this.alternates.put(family, alternate);
      }
docs: Adds a font family mapping (if the specified alternate is
  {@code null} it has the effect of clearing any existing mapping).
  
  @param family  the font family name ({@code null} not permitted).
  @param alternate  the alternate ({@code null} permitted).