code: @Deprecated
     // TODO change to getMergedFilter
      public UnicodeSet getFilterAsUnicodeSet(UnicodeSet externalFilter) {
          if (filter == null) {
              return externalFilter;
          }
          UnicodeSet filterSet = new UnicodeSet(externalFilter);
          // Most, but not all filters will be UnicodeSets.  Optimize for
          // the high-runner case.
          UnicodeSet temp;
          try {
              temp = filter;
          } catch (ClassCastException e) {
              filter.addMatchSetTo(temp = new UnicodeSet());
          }
          return filterSet.retainAll(temp).freeze();
      }
docs: Returns the intersectionof this instance's filter intersected with an external filter.
  The externalFilter must be frozen (it is frozen if not).
  The result may be frozen, so don't attempt to modify.
  @deprecated  This API is ICU internal only.
  @hide draft / provisional / internal are hidden on Android