According to: http://docs.oracle.com/javase/tutorial/java/javaOO/lambdaexpressions.html#lambda-expressions-in-gui-applications
Previously:
btn.setOnActio
For scala versions 2.12 onward support comes out of the box.
Lambdas was introduced in java language and has a little in common with scala functions. They're compiled down to a different bytecode, has different hierarchy (scala functions were here long before and apparently java designers have chosen clean room implementation without compatibility with scala).
Currently support is pretty much limited and what you're trying to do is not possible (out of the box):
The Scala 2.11 series targets Java 6, with (evolving) experimental support for Java 8. In 2.11, Java 8 support is mostly limited to reading Java 8 bytecode and parsing Java 8 source. We will be expanding Scala's (experimental) Java 8 support and interop throughout the 2.11 series. - See more at: https://typesafe.com/blog/scala-211-has-arrived#sthash.ukr4FSpU.dpuf
There is ongoing efforts to resolve this problem
See also discussion on scala roadmap to support java 8 functions.