Build expects CFR decompiler as a cfr.jar and FernFlower decompiler as a fernflower.jar provided at lib/ directory.
NB: I had to disable tests to build FernFlower, as there was one failing.
There is a Java class provided to reverse engineer its Scala original class, knowing that the Java one was obtained by decompilation. If anything, all what was expressed is in the Java class file
The process of completing the challenge is documented in the process file with further links.
Results are HashPartitioning.scala and other classes in the src directory that were reconstructed to the best deducible state, using only the provided Java class and no knowledge of Apache Spark, as declared per task definition.
Company's reviewer didn't know how to evaluate thought process, be aware to provide what they think they asked you to do by that task