Error: Compile Error: Method does not exist or incorrect signature: [String].add(String)

Question

trigger TriggerOpportunityBudget on Opportunity(after insert, after update) {
    Map < Id, String > mapAcctIdOppList = new Map < Id, String > ();
    set < Id > AccId = new set < Id > ();
    for (Opportunity Opp: (List < Opportunity > ) trigger.new) {
        AccId.add(Opp.AccountId);
    }

    List < Opportunity > listOpp = [SELECT Budget__c, AccountId 
                                      FROM Opportunity where AccountId in : AccId];

    for (Opportunity opp2: listOpp) {
        if (String.isNotBlank(opp2.AccountId)) {
            if (!mapAcctIdOppList.containsKey(opp2.AccountId)) {
                mapAcctIdOppList.put(opp2.AccountId, opp2.Budget__c);
            }
            mapAcctIdOppList.get(opp2.AccountId).add(opp2.Budget__c);
        }
    }
    system.debug(mapAcctIdOppList);
}

Answer 1

So there is your map is wrong defined I guess that should be

map<Id, List<String>> = new map<Id, List<String>>();

And in you code when you adding into this map

if(!mapAcctIdOppList.containsKey(opp2.AccountId)) {
       mapAcctIdOppList.put(opp2.AccountId, new list<String>{opp2.Budget__c });
 }
else{
    mapAcctIdOppList.get(opp2.AccountId).add(opp2.Budget__c );
}

This way it should work..