Android Code Snippet Desugar alternative for conflictive jars

Hi all,

If you've ever had to desugar some libraries that use Java 8 features, there's a great B4J utility that @Erel posted HERE.
It does the job after following the instructions, adding the needed dependencies, and outputs a desugared Merged.jar

However, there are some specific libraries that seem to use some (more recent?) Java 8 features where the desugarer fails to work with. See HERE and HERE.

After spending lots of hours trying to find a solution, I finally could manage it (there may be other ways. This has worked for me, and may work or fail in other cases). --> So I decided to post the relevant clues in case someone also runs into the same situation. If this is the case, please give feedback

(Note: this procedure does NOT replace the whole B4J's utility desugaring process -unpacking aars, merging classfiles, desugaring...- but is intended to individually deal with the 'conflictive' .jar class files)

How to proceed:
- Identify the conflictive jar file (the one that outputs a similar error as reported in the linked threads above, or perhaps others)
- Download an updated r8.jar from the maven repository. I used r8-2.1.96.jar
- (Starting from some version, R8-D8 includes a non-official but functional support to produce .jar class outputs, instead of DEX)
- Place them in the same folder to simplify things
- Execute the following line (note: it's just a use-case example, broken into lines for clarity. Adapt it to your specific use case)

java -cp r8-2.1.96.jar                            '<-- specify D8 classpath entry to r8-2.1.96.jar (or the version you have)
  --output C:\Develop\B4J-Desugar\tests\classes2.jar                            '<-- path+name of the desired output .jar
  C:\Develop\B4J-Desugar\tests\classes.jar                                               '<-- path+name of the input jar
   --lib C:\Develop\Basic4android\B4A_AndroidSDK\platforms\android-30\android.jar       '<-- Path to android.jar
   --classpath C:\Develop\Basic4android\B4A_AndroidSDK\extras\b4a_remote\androidx\core\core\1.3.1\unpacked-core-1.3.1\jars\classes.jar         '<-- Add one line like this for each dependency
   --classpath C:\Develop\Basic4android\B4A_AndroidSDK\extras\b4a_remote\androidx\lifecycle\lifecycle-common\2.2.0\lifecycle-common-2.2.0.jar
   --classpath C:\Develop\Basic4android\B4A_AndroidSDK\extras\b4a_remote\androidx\lifecycle\lifecycle-extensions\2.2.0\lifecycle-extensions-2.2.0\classes.jar 
   --classpath C:\Develop\Basic4android\B4A_AndroidSDK\extras\b4a_remote\androidx\lifecycle\lifecycle-process\lifecycle-process-2.3.1\classes.jar
   --classfile                                                                                   '<--Note the last --classfile option at the end. If it is not added. it will output a compiled .dex file instead of a .jar

Good luck with it! :)