1. Scalagen - Java to Scala Conversion
Scalagen was born out of the idea that we would like to port some of our projects from Java to Scala source code. Instead of porting all the code manually a tool could be used to do most of the bulk work. We did some initial experiments with Jatran, but found it lacking.

After that we investigated some options to write such a tool ourselves. First we needed a parser for Java sources to turn into abstract syntax trees. We decided to use the Javaparser framework since it supports Java 5 source and had an easy to use API.

2. Google jatran

jatran
A Source to Source transformer, from Java to Scala

3.  Scala to Java  Conversion

You have to decompile the .class files to java source files, beware that scala produces many more .class files

You can use javap from oracle included in the sdk or jad decompiler

for detailed explanation read the following article:

Link scala class to java source: https://alvinalexander.com/scala/scala-class-to-decompiled-java-source-code-classes