How can I get the value of the dependent picklist ?

You can use Metadata API via REST to get states. In my knowledge, we cant directly query it in apex.

https://success.salesforce.com/ideaView?id=08730000000keCTAAY

Hii Piyush,

This is the scenario for dependent picklist.When you click on country picklist and select india than you will get related states to india in states picklist field and when you ll choose any state than you will get related cities to that state.

code is here :

Visualforce Page:

<apex:page controller="sample">
<apex:form >
<apex:pageBlock >
<apex:pageBlockSection columns="2">
<apex:pageBlockSectionItem >
<apex:outputLabel value="country"/>
</apex:pageBlockSectionItem>
<apex:selectList size="1" value="{!country}">
<apex:selectOptions value="{!countries}"/>
<apex:actionSupport event="onchange" reRender="a"/>
</apex:selectList>
<apex:pageblockSectionItem >
<apex:outputLabel value="state"/>
</apex:pageblockSectionItem>
<apex:pageblockSectionItem >
<apex:selectList size="1" value="{!state}" id="a">
<apex:selectOptions value="{!states}"/>
<apex:actionSupport event="onchange" reRender="b"/>
</apex:selectList>
</apex:pageblockSectionItem>
<apex:pageblockSectionItem >
<apex:outputLabel value="city"/>
</apex:pageblockSectionItem>
<apex:pageblockSectionItem >
<apex:selectList size="1" value="{!city}" id="b">
<apex:selectOptions value="{!cities}"/>
</apex:selectList>
</apex:pageblockSectionItem>
</apex:pageBlockSection>
</apex:pageBlock>

</apex:form>

</apex:page>

Controller:

public class sample
{
public String country{get;set;}
public String state {get;set;}
public String city {get;set;}

public List<SelectOption> getCountries()
{
List<SelectOption> options = new List<SelectOption>();
options.add(new SelectOption('None','--- None ---'));
options.add(new SelectOption('US','USA'));
options.add(new SelectOption('IN','India'));
options.add(new SelectOption('AUS','Australia'));
return options;
}

public List<SelectOption> getStates()
{
List<SelectOption> options = new List<SelectOption>();
if(country=='US')
{
options.add(new SelectOption('NYK','NewYork'));
options.add(new SelectOption('WSTN','Washington'));
}
else if(country=='IN'){
options.add(new SelectOption('DLI','Delhi'));
options.add(new SelectOption('MUM','Mumbai'));
}
else if(country=='AUS'){
options.add(new SelectOption('SYD','Sydney'));
options.add(new SelectOption('MLB','Melburn'));
}
return options;
}
public List<SelectOption> getCities()
{
List<SelectOption> options = new List<SelectOption>();
if(country == 'US')
{
if(state == 'NYK'){
options.add(new SelectOption('C1','City1'));
options.add(new SelectOption('C2','City2'));
}
else if(state == 'WSTN')
{
options.add(new SelectOption('C3','City3'));
options.add(new SelectOption('C4','City4'));
}
}
else if(country == 'IN')
{
if(state== 'DLI'){
options.add(new SelectOption('C5','City5'));
options.add(new SelectOption('C6','City6'));
}
else if(state=='MUM'){
options.add(new SelectOption('C7','City7'));
options.add(new SelectOption('C8','City8'));
}
}
else if(country=='AUS'){
if(state=='SYD'){
options.add(new SelectOption('C9','City9'));
options.add(new SelectOption('C10','City10'));
}
else if(state=='MLB'){
options.add(new SelectOption('C11','City11'));
options.add(new SelectOption('C12','City12'));
}
}

return options;
}
}

i hope this code will help you.