code: public MultimapJoiner useForNull(final String nullText) {
  		return new MultimapJoiner(entryJoiner.useForNull(nullText), separator, keyValueSeparator) {
  			@Override
  			protected CharSequence toString(Object part) {
  				return (part == null) ? nullText : MultimapJoiner.this.toString(part);
  			}
  
  			@Override
  			public MultimapJoiner useForNull(String nullText) {
  				throw new UnsupportedOperationException("already specified useForNull");
  			}
  
  			@Override
  			public MultimapJoiner skipNulls() {
  				throw new UnsupportedOperationException("already specified useForNull");
  			}
  		};
  	}
docs: Returns a multimap joiner with the same behavior as this one, except automatically substituting {@code nullText} for
  any provided null keys or values.