Scala sits on top of the JVM. It seamlessly integrates with existing Java code and has many features that Java doesn't.
Scala syntax is flexible making it one of the best language to implement DSLs (Domain Specific Language). This is particularly appreciated in our case to implement modeling layers for optimization (more on Scala syntax flexiblity for DSL implementation here).
Scala is efficient (a paper of Google comparing Java, Scala, C++ and Go)
Resources to learn Scala
In case you want to learn more about Scala, we found this tutorial made by twitter very good (yes part of twitter is implemented with Scala).