Scala, like most/all languages that run bolted on top of the JVM can use almost all of the libraries that Java itself can use, since the Scala code gets auto-converted to Java classes. (Typically (in the examples I've seen) one Scala class gets converted into many Java classes)

This is not entirely dissimilar from the way that JSP gets converted into a Servlet.

Scala/AlternateLanguageRunningOnJVM fans please provide corrections as appropriate, but be gentle, I'm making some broad generalisations here with all the flaws implied by that methodology. :D