code: public StorageTierView getTierView(String tierAlias) {
      StorageTierView tierView = mAliasToTierViews.get(tierAlias);
      if (tierView == null) {
        throw new IllegalArgumentException(
            ExceptionMessage.TIER_VIEW_ALIAS_NOT_FOUND.getMessage(tierAlias));
      } else {
        return tierView;
      }
    }
docs: Provides {@link StorageTierView} given tierAlias. Throws an {@link IllegalArgumentException} if
  the tierAlias is not found.
  
  @param tierAlias the alias of this tierView
  @return the {@link StorageTierView} object associated with the alias