Method does not exist or incorrect signature: [String].put(String)

Question

Can someone explain why I'm getting this error? I'm trying to retrieve the value of 'ownerIdstoNames' map with a key of type Id but the error says that I'm using a String?

map<id, Case> caseOldMap = (map<id, Case>) trigger.oldMap;
map<id, Case> caseNewMap = (map<id, Case>) trigger.newMap; 
Map < Id, String > ownerIdstoNames = new Map < Id, String > ();

 for (Case c: cases) {
     if (caseNewMap.get(c.Id).OwnerId != caseOldMap.get(c.Id).OwnerId) {
         ownerIdstoNames.put(caseNewMap.get(c.Id).OwnerId, ' ');
     }
 }
 List < User > owners = [SELECT Name, Id FROM User WHERE Id IN: ownerIdstoNames.keyset()];
 for (User owner: owners) {
     ownerIdstoNames.get(owner.Id).put(owner.Name); // Method does not exist or incorrect signature: [String].put(String)
 }

Answer 1

The problem is not while you are retrieving; it's when you are assigning the value.

ownerIdstoNames.get(owner.Id).put(owner.Name); // Method does not exist or incorrect signature: [String].put(String)

Lets break this into two steps:

1. ownerIdstoNames.get(owner.Id)

   This will return a String because you have a map

2. String.put(owner.Name)

   There is no string method called put .Map has a put method but not String .

   Instead just use put on the map and because the key of the map is unique ,your key will have correct value assigned

   for (User owner: owners) {
       ownerIdstoNames.put(owner.Id,owner.Name); //Put into the map
   }