java - Placing average cost of something into a table -

i've been trying add average cost of petshop 6th slot of table , can't work. no errors appear , i'm totally lost on how so. below method calculates average cost of pets in shop.

public string getcalculatepetshopaverage(string shop)         {             arraylist<pets> petshoppets = new arraylist<pets>();             arraylist<pets> petslist = new arraylist<pets>();              petslist.addall(dogs);             petslist.addall(fishes);               for(pets pet : petslist)             {                if(pet != null && pet.getshop().equals(shop))                 {                    petshoppets.add(pet);                }             }              double totalaverage = 0;             int = 0;             for(pets pet : petshoppets)             {                i++;                totalaverage = totalaverage + pet.getprice();             }             system.out.println("there "+i+" pets in here");                this.averagecost = totalaverage;             system.out.println(""+this.averagecost);              return string.format("%.2f",totalaverage/i);           } 

below part of code table i'm displaying in gui. i've tried calling method name of petshop whenever run gui application, in 6th section of table can see nan.

@override public object getvalueat(int rowindex, int columnindex)  {     pets pets = petshops.get(rowindex);      switch (columnindex)     {         case 0:             return pets.getshop();         case 1:             return pets.gettype();         case 2:              return pets.getprice();         case 3:             return pets.getdateacquired();         case 4:             return pets.getnotes();         case 5:             return pets.getcalculatepetshopaverage(pets.getshop());      }     return null;  } 

i'm still quite new java , have no idea how add average cost table, i'd appreciate help.