Hello,
I have 3 objects zipcode , address, contact.
---contact have one physical address & contact have firem channel 
---address have one zipcode(text)
---zipcode have zip and territory channel 

i am facing de reference null object please suggest me if any one knows and find below my code....

Public void populateZoneZipTerritory(List<Contact> contactList , Map<Id, Contact> contactOldMap){
        set<Id> contId = new set<Id>();
        set<string> zipcodeName = new set<string>();
        set<string> firmChannel = new set<string>();
        Map<string, Zip_Code__c> zipcodeMap = new Map<string, Zip_Code__c>();
        Map<Id, Contact> contMap = new Map<Id, Contact>();
        for(Contact c : contactList){
            if((c.Firm_Channel__c != null)&&(c.Firm_Channel__c != contactOldMap.get(c.Id).Firm_Channel__c)){
                contId.add(c.Id);
                firmChannel.add(c.Firm_Channel__c);
                contMap.put(c.Id, c);
                system.debug('*******'+contMap);
            }
        }
        List<Addresses__c> addressList =  new List<Addresses__c>([select Id, Zip_Code__c, Contact__c, Address_Type__c from Addresses__c where Address_Type__c = 'Physical' and Contact__c IN :contId]);
        for(Addresses__c a: addressList){
            zipcodeName.add(a.Zip_Code__c);
        }
        List<Zip_Code__c> zipcodeList = new List<Zip_Code__c>([select Id,Name,Zone__c,Zone__r.Territory__c,Territory_Channel__c from Zip_Code__c where Name IN :zipcodeName and Territory_Channel__c IN :firmChannel]);
        for(Zip_Code__c z :zipcodeList){
            zipcodeMap.put(z.Name+''+z.Territory_Channel__c, z);
            system.debug('******'+ zipcodeMap);
        }
        Map<Id, Zip_Code__c> conZipMap = new Map<Id, Zip_Code__c>();       
        for(Contact c: contactList){
            for(Addresses__c a: addressList){
                if(a.Contact__c == c.Id){
                    if(zipcodeMap.containsKey(string.valueOf(a.Zip_Code__c+''+c.Firm_Channel__c))){
                        conZipMap.put(a.Contact__c ,  zipcodeMap.get(a.Zip_Code__c+''+c.Firm_Channel__c));                        
                        system.debug('&&&&&' + conZipMap);
                        system.debug('&&&&&' + a.Zip_Code__c+''+c.Firm_Channel__c);--------------> // i got the value here
                        system.debug('^^^^^^^^'+conZipMap.containsKey(c.Zip_Code__c));
                    }
                } 
            }
        }
        for(Contact c: contactList){
            if(!conZipMap.containsKey(c.Zip_Code__c)){
                if((c.Firm_Channel__c != null)&&(c.Firm_Channel__c != contactOldMap.get(c.Id).Firm_Channel__c)){
                    c.Zip_Code__c = conZipMap.get(c.Zip_Code__c).Id;------------------->// i need to update zipcode here
                    system.debug('&&&&&&&' + c.Zip_Code__c);
                    system.debug('&&&&&&&' + conZipMap.get(c.Zip_Code__c).Id);
                }
            }
        }
    }