I never think the experts'result to my own.I respects the experts' projects for only studying and inviting Thai programmers are interested in B4X. I don't have a profit fromB4X., I do learn B4X because I do love Erel's B4X.
When a member shared the jar file or apk file, it is mean to be use as it-is. It should not be modified further.
It also means the source code is NOT AVAILABLE as Open Source.
Decompiling or Repackage it is against the law, rules, etiquette or moral. You are not respecting the work of the developer.
Even if it is for non-profit, permission must be requested from the original developer. He/She may allow you to do what you are requesting.
Unless it is clearly stated in a post by the developer that you are allowed to modify and use it in production and sell for profit.
If you are sure it is compiled using Java 8 then run it using Java 8.