How to trigger ajax and update another dropdown when user select dropdown - ajax

Question

I am implementing a dropdown change event and on change event, another dropdown should update the value based on selected dropdown value.
I tried with my actual code and don't work. later I tried Primeface sample dropdown ajax example which also did not work.
Primeface Version: 7.0 (tried 6.1 also)
Java : 1.8
dropdown.xhtml
<h:form>
<p:growl id="msgs" showDetail="true" />
<p:panel header="Select a Location" 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">
<p:ajax listener="#{dropdownView.onCountryChange}" update="city" />
<f:selectItem itemLabel="Select Country" itemValue="" noSelectionOption="true" />
<f:selectItems value="#{dropdownView.countries}" />
</p:selectOneMenu>
<p:outputLabel for="city" value="City: " />
<p:selectOneMenu id="city" value="#{dropdownView.city}" style="width:150px">
<f:selectItem itemLabel="Select City" itemValue="" noSelectionOption="true" />
<f:selectItems value="#{dropdownView.cities}" />
</p:selectOneMenu>
</h:panelGrid>
<p:separator />
<p:commandButton value="Submit" update="msgs" action="#{dropdownView.displayLocation}" icon="pi pi-check" />
</p:panel>
</h:form>```
dropdownView.java
#ManagedBean
#ViewScoped
public class DropdownView implements Serializable {
private Map<String,Map<String,String>> data = new HashMap<String, Map<String,String>>();
private String country;
private String city;
private Map<String,String> countries;
private Map<String,String> cities;
#PostConstruct
public void init() {
countries = new HashMap<String, String>();
countries.put("USA", "USA");
countries.put("Germany", "Germany");
countries.put("Brazil", "Brazil");
Map<String,String> map = new HashMap<String, String>();
map.put("New York", "New York");
map.put("San Francisco", "San Francisco");
map.put("Denver", "Denver");
data.put("USA", map);
map = new HashMap<String, String>();
map.put("Berlin", "Berlin");
map.put("Munich", "Munich");
map.put("Frankfurt", "Frankfurt");
data.put("Germany", map);
map = new HashMap<String, String>();
map.put("Sao Paulo", "Sao Paulo");
map.put("Rio de Janerio", "Rio de Janerio");
map.put("Salvador", "Salvador");
data.put("Brazil", map);
}
public Map<String, Map<String, String>> getData() {
return data;
}
public String getCountry() {
return country;
}
public void setCountry(String country) {
this.country = country;
}
public String getCity() {
return city;
}
public void setCity(String city) {
this.city = city;
}
public Map<String, String> getCountries() {
return countries;
}
public Map<String, String> getCities() {
return cities;
}
public void onCountryChange() {
if(country !=null && !country.equals(""))
cities = data.get(country);
else
cities = new HashMap<String, String>();
}
public void displayLocation() {
FacesMessage msg;
if(city != null && country != null)
msg = new FacesMessage("Selected", city + " of " + country);
else
msg = new FacesMessage(FacesMessage.SEVERITY_ERROR, "Invalid", "City is not selected.");
FacesContext.getCurrentInstance().addMessage(null, msg);
}
}```
Expected is on selecting any country, city should load into city dropdown.