java - Why must Lucene Token Filter classes be declared "final"? -

according solr's blog, lucene requires tokenfilter subclasses must declared final. advantage of final classes in context? makes impossible extend functionality of existing tokenfilter.

tokenfilter extends tokenstream, , javadoc says:

the tokenstream-api in lucene based on decorator pattern. therefore non-abstract subclasses must final or have @ least final implementation of incrementtoken()! checked when java assertions enabled.

for example, standardfilter not marked final (only incrementtoken() is).

so if want extend existing tokenfilter it's best via delegation.