If you look at the code, it will be obvious.
A ClassCastException will occur when doing operations like:
aLocatorSet = (TreeSet) ((Map.Entry) aValueIterator.next()).getValue();

That is because the Iterator being used is from a Collection, not a map.
The change is to get the Iterator from the entrySet() of the TreeMap instead
of the values(). e.g.
aValueIterator = itsValues.entrySet().iterator();
instead of
aValueIterator = itsValues.values().iterator();

If you run the buggy version, everything appears OK (poor error handling) but
indexes are not used.

Patch for org.apache.xindice.core.indexer.MemValueIndexer is attached.