Alternate.cls

// Alternate way to get the name from a given Id
String objName = objId.getSObjectType().getDescribe().getName();

ObjectPrefix.cls

  /**
  * Gets a map of the prefix for an object to the object name
  *
  * @return A map of id prefix to object name
  */
  public static Map<String, String> getObjectToPrefixMap() {
      Map<String, Schema.SObjectType> gd = Schema.getGlobalDescribe();
      Map<String,String> keyPrefixMap = new Map<String,String>{};
      Set<String> keyPrefixSet = gd.keySet();
      for(String sObj: keyPrefixSet){
          Schema.DescribeSObjectResult r =  gd.get(sObj).getDescribe();
          String tempName = r.getName();
          String tempPrefix = r.getKeyPrefix();
          keyPrefixMap.put(tempPrefix,tempName);
      }

      return keyPrefixMap;
  }