trigger listOfContactsOnAccount on contact (after insert , after update,after delete){ 
  set<id> accountIdSet = new set<id>();
  
  if( trigger.isInsert|| trigger.isUpdate){
    for(contact c : trigger.new){
      accountIdSet.add(c.AccountId);
    }
  }

  if(trigger.isDelete){
    for(contact c: trigger.old){
      accountIdSet.add(c.AccountId);
    }
  }
            
  //2.create map
  List<contact> cont = [ Select AccountId, Address_for_contact__c from contact where AccountId IN : accountIdSet ]; 
    
    //using a map of lists instead of a map of contacts
    Map<id,List<contact>> accountContactsMap = new map<id, List<contact>>();
    for(contact c : cont){
      //check if a entry in the map exists for the account
      if(!accountContactsMap.containsKey(c.accountId)){
        //if it doesnt then create one
        accountContactsMap.put(c.accountId, new List<Contact>());
      }
      //get the contact list for the account and add the contact in
      accountContactsMap.get(c.accountId).add(c);
    }
  
    List<Account> accountsToUpdate= new List<Account>();
    string l='';
    
              for(contact cn :trigger.new)
   {
     if(accountContactsMap != null)
         { 
              list<contact> aci = accountContactsMap.get(cn.AccountId);  
              account ac = new account(id=cn.AccountId);
                  {
                    for(contact c : aci)
                         
                         { if(l==null)
                          
                              {
                              l =   c.Address_for_Contact__c ;        
                              }
                             else
                             {
                             l= l+','+c.Address_for_contact__c;
                             }
                         }     
                             
               
                          ac.Final_address_of_contacts__c = l;
                          accountsToUpdate.add(ac);
                    }  
              
          }
          
   }
 update accountsToUpdate;  
}

@isTest(SeeAllData=true)
public class testlistOfContactsOnAccount
{
public static testMethod void accountaddress()
{


account acc = new account();
acc.Name='account name';
//acc.Final_address_of_contacts__c ='agsah,bawdgh';
insert acc;


List<Contact> clist = new List<Contact>();
//id accid=acc.id;
contact ct1 = new contact(LastName='test1',AccountId = acc.Id);
//ct1.Accountid=accid;
//ct1.LastName='abc';
ct1.Address_for_contact__c='a';
clist.add(ct1);

contact ct2 = new contact(LastName='test2',AccountId = acc.Id);
//ct2.AccountId=accid;
//ct2.LastName='xyz';
ct2.Address_for_contact__c='b';
clist.add(ct2);
//ct2.accountid=acc.id
//test.startTest();


insert clist;

//string l = null;
//for(contact c : cList)
//{
 //if(l == null)
 //{ l= c.Address_for_contact__c;
 
// }
// else
// {
// l=l+','+c.Address_for_contact__c;
// }
//}


//acc.Final_address_of_contacts__c=l;
//update acc;
//test.stopTest();

account updt =[ select id,Final_address_of_contacts__c from account where id=:acc.id];
system.assertEquals('a,b', updt.Final_address_of_contacts__c);

//List<Contact> deleteList = new List<Contact>();

  //  deleteList.add(ct1);
  //  delete deleteList;
    
//account updt1 =[ select id,Final_address_of_contacts__c from account where id=:acc.id];
//system.assertEquals('karol bagh', updt1.Final_address_of_contacts__c);

  }
  }