Scala is a hybrid Object Oriented & Functional programming language, which means you can program in OO style or you can program in a purely Functional style.The Scala website has a A Scala Tutorial for Java Programmers.

Start with the excellent free Coursera mooc Functional Programming Principles In Scala & Principles of Reactive Programming by Scala creator Martin Odersky. This course runs every year and is a great place to get started with FP in Scala, also see Functional programming for more on functional programming.

Quick References

Java is far more widely used than Scala, and probably always will be. But in the emerging world of Big Data, distributed processing and scalability. Tools like Kafka (messaging), Spark (distributed analytics) and Akka (actor-based concurrency) are all written in Scala. There are Java APIs for all of these but the Scala API for Spark is much easier to use than the Java API. A lot of this stuff is increasingly based on ideas from the Reactive Manifesto, and Typesafe (the company behind Scala).