The last thing I know was they moved from Ruby to Scala. Did they move to Java without me noticing or do you mean Scala by Java (since it's a JVM language too)?
EDIT: according to Wikipedia the service itself moved from Ruby to Scala while search moved from Ruby to Java.
Twitter was Ruby but moved to Scala and Java :D
Twitter's frontend is still all Ruby/Rails. The Ruby -> Java claim is common, but incorrect. (Source: I know multiple engineers in SF who work there.)