primefaces binding , required attribute for multiSelectListbox not triggred - jsf

Question

I want to create a JSF application. In the application the user will have two drop downs.
If a user selects the country Mexico / value 3 from the first drop down then he is required to choose the option Cancun / value 6 from the multiselect
I have added the binding , required attribute for multiSelectListbox but they are not triggered when the drop down is selected
<h:form>
<p:growl id="msgs" showDetail="true" skipDetailIfEqualsSummary="true" />
<p:panel header="Tranfer Destination" style="margin-bottom:10px;">
<h:panelGrid columns="2" cellpadding="5">
<p:outputLabel for="country" value="Country: " />
<p:selectOneMenu id="country" value="#{dropdownView.country}" style="width:150px" required="true" binding="#{country}">
<f:selectItem itemLabel="Select Country" itemValue="" noSelectionOption="true" />
<f:selectItems value="#{dropdownView.countries}" />
</p:selectOneMenu>
<p:outputLabel for="city" value="City: " />
<p:multiSelectListbox id="city" value="#{dropdownView.city}" style="width:150px" required="#{not empty param[country.6]}">
<f:selectItem itemLabel="Select City" itemValue="" noSelectionOption="true" />
<f:selectItem itemLabel="New York" itemValue="1"> </f:selectItem>
<f:selectItem itemLabel="Chicago" itemValue="2"> </f:selectItem>
<f:selectItem itemLabel="Seattle" itemValue="3"> </f:selectItem>
<f:selectItem itemLabel="Toronto" itemValue="4"> </f:selectItem>
<f:selectItem itemLabel="Ontario" itemValue="5"> </f:selectItem>
<f:selectItem itemLabel="Cancun" itemValue="6"> </f:selectItem>
<f:selectItem itemLabel="Tijuana" itemValue="7"> </f:selectItem>
</p:multiSelectListbox>
</h:panelGrid>
<p:separator />
<p:commandButton value="Submit" update="msgs" action="#{dropdownView.displayLocation}" icon="pi pi-check" />
</p:panel>
</h:form>
The option Cancun / value 6 is required from the second drop down only If the user selects the country Mexico / value 3 from the first drop down

Answer 1

After selecting first drop-down, you need to update <p:multiSelectListbox id="city"/>.
<p:selectOneMenu id="country"
value="#{dropdownView.country}"
style="width:150px"
required="true"
binding="#{country}">
<f:selectItem itemLabel="Select Country" itemValue="" noSelectionOption="true" />
<f:selectItems value="#{dropdownView.countries}" />
<p:ajax update="city"/>
</p:selectOneMenu>