problem:lucene did not delete old index file after optimize method called

[luocan19826164]

lucene did not delete old index file after optimize method called.
ps:I call IndexWriter.getReader() and then call old IndexReader.close() every 1 
minute,
a long time pass, I watche old index file did not disappear.
after I restart my program, optimize index file disappear,but old index file 
stay!!