複数のSharedPreferenceのStringデータをsharedPreferenceに保存するには？（アンドロイド）

   public  String[] getFavoriteList(Activity activity){
            String favoriteList = getStringFromPreferences(activity,null,"favorites");
            return convertStringToArray(favoriteList);
        }

     public  boolean addFavoriteItem(Activity activity,String favoriteItem){
            //前のデータを取得しる
            String favoriteList = getStringFromPreferences(activity,null,"favorites");
            // 新しく追加処理を行う
            if(favoriteList!=null){
                favoriteList = favoriteList+","+favoriteItem;
            }else{
                favoriteList = favoriteItem;
            }
            // Shared Preferencesに保存する
            return putStringInPreferences(activity,favoriteList,"favorites");
        }


  public String getStringFromPreferences(Activity activity,String defaultValue,String key){
            SharedPreferences sharedPreferences = activity.getPreferences(Activity.MODE_PRIVATE);
            String temp = sharedPreferences.getString(key, defaultValue);
            return temp;
        }
   public  boolean putStringInPreferences(Activity activity,String nick,String key){
            SharedPreferences sharedPreferences = activity.getPreferences(Activity.MODE_PRIVATE);
            SharedPreferences.Editor editor = sharedPreferences.edit();
            editor.putString(key, nick);
            editor.commit();
            return true;
        }

    //文字を配列に変更する
        private  String[] convertStringToArray(String str){
            String[] arr = str.split(",");
            return arr;
        }