code: public UriComponentsBuilder fragment(String fragment) {
  		if (fragment != null) {
  			Assert.hasLength(fragment, "'fragment' must not be empty");
  			this.fragment = fragment;
  		}
  		else {
  			this.fragment = null;
  		}
  		return this;
  	}
docs: Set the URI fragment. The given fragment may contain URI template variables,
  and may also be {@code null} to clear the fragment of this builder.
  @param fragment the URI fragment
  @return this UriComponentsBuilder