And does that mean we can develop a Zebra application in Java?
Zebra development has started as a porting of Java (Light-Weight Visual Library) project. For this purpose a special Java to JS converter was implemented. The initial idea was not a building tool that helps to write code in both Java and JS languages, but speed up moving Java code base into JS world. Since that time zebra JS (when it was clear that building UI on HTML Canvas is feasible) was actually re-written and re-organized to follow the rules JS dictated.
Nevertheless, if your target is to write code in java than translate it in zebra JS you can try to adjust the converter for your needs. The converter is written in Ruby using very nice TreeTop library that helps to define your parser. Let me know you need source code.