Controller.java

if (len <= 0) {
   FacesContext.getCurrentInstance().addMessage(null, new FacesMessage("You must enter a temperature"));
}

index.xhtml

<div id="container">
		<h:form id="convertForm">
			<h:panelGrid columns="4">
				<h:outputLabel for="tempFrom">Temperature:</h:outputLabel>
				<h:inputText value="#{tempFrom}" id="tempFrom" />
                                <h:message for="tempFrom" errorClass="invalid" />
				<h:commandButton id="add" value="Convert"
					action="#{tempConvert.setTempFrom(tempFrom)}">
				</h:commandButton>
			</h:panelGrid>
		</h:form>
        <h:form id="response">
            <h:panelGrid columns="2">
                <h:outputLabel>Converts to:</h:outputLabel>
                <h:outputLabel value="#{tempConvert.temperature}" />
            </h:panelGrid>
        </h:form>
	</div>