how to covert object dataType into decimal dataType.

Hi,

you can use this

public static void NewcountyUpdate(List<id> countyIds){

List<New_Company__c> newval = new List<New_Company__c>();

for (AggregateResult ar : [select count(Id) totalCount, Company_Realign__c countyId From Contact where Company_Realign__c = :countyIds GROUP BY Company_Realign__c ] ){
newval.add(new New_Company__c(
Id = (Id) ar.get('countyId'),

//This One
Alumni_contact_count__c = (Decimal) ar.get('totalCount')
));
}
update newval;
}