code: public int getIndex(T value) {
          int count = getItemCount();
          for (int i = 0; i < count; i++) {
              if (Objects.equals(getValue(i), value)) {
                  return i;
              }
          }
          return -1;
      }
docs: Gets the index of the specified value.
  
  @param value the value of the item to be found
  @return the index of the value