<p><pre><code> class ListingCache extends CacheObserver
</code></pre>
The rest of the code is Ruby but this isn't. Guessing this code wasn't copied and pasted but written on the fly by someone who's been doing some Java recently? :-)
<p><pre><code> "There are only two hard problems in Computer Science:
cache invalidation and naming things."
-- Phil Karlton
</code></pre>
Always helpful to see how someone else is solving the problem at scale.