Ask HN: Technologies to use for a pet project

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I have been out of side projects for a while and been only working at Java (server side). Hence I have no idea what new technology is out. So to learn new technology + build a pet project I need your suggestion.

The project I want to build requires

- A neat UI (I like Vaadin but not sure how good it is) - Cross platform UI compatibility - Server Side to do transactions, printing on different machines, Roles management etc

Thanks.