code: private void addMandatorySelectExample() {
  		add(new WHeading(HeadingLevel.H3, "Mandatory WRadioButtonSelect"));
  
  		add(new ExplanatoryText("When a WRadioButtonSelect is mandatory it needs a visible labelling element, otherwise many users may not know that "
  				+ "the component requires an answer."));
  		final WRadioButtonSelect select = new WRadioButtonSelect("australian_state");
  		select.setButtonLayout(WRadioButtonSelect.LAYOUT_FLAT);
  		select.setMandatory(true);
  		add(new WLabel("Mandatory selection", select));
  		add(select);
  
  		add(new WHeading(HeadingLevel.H3, "Mandatory WRadioButtonSelect in a WFieldLayout"));
  		WRadioButtonSelect select2 = new WRadioButtonSelect("australian_state");
  		select2.setButtonLayout(WRadioButtonSelect.LAYOUT_FLAT);
  		select2.setMandatory(true);
  		final WFieldLayout layout = new WFieldLayout();
  		layout.setLabelWidth(25);
  		add(layout);
  		layout.addField("Required selection", select2).getLabel().setHint("Required");
  		select2 = new WRadioButtonSelect("australian_state");
  		select2.setButtonLayout(WRadioButtonSelect.LAYOUT_FLAT);
  		select2.setMandatory(true);
  		select2.setToolTip("Select a state");
  		layout.addField((WLabel) null, select2);
  	}
docs: adds a WRadioButtonSelect with setMandatory(true).