Groovy is awesome.
Groovy is a language based on Java with much greater ease of use as well as interactive REPL and scripting.
If I'd known Groovy in 2013 I'd have never written anything in Jython as Python constantly breaks code every few years.
This is easily one of my top pick languages. The more you use Groovy, the less you'll want to go back to Java.
Groovy is the language of Jenkins.
If you want to do anything advanced in Jenkins, you need to know Groovy.
Jenkins has an interactive console in which you can execute snippets of Groovy (there are a couple examples in between the shell scripts here).
See here for substantial Groovy code that I used in production for years:
Partial port from private Knowledge Base page 2016+