Something I use a lot in my off time is this Android app https://play.google.com/store/apps/details?id=com.sololearn. It is relaxing, and easy to pick up while designing the next step in your application. They probably have an iOS app too, if you like. It is only for basic language information, but it will get you started on almost any well known language. The only problem I have seen is that they do not teach you anything about compilers, only language syntax. They have their own web-based compilers to test snippets of code, which is sometimes far removed from real-world application.
I also use QT a lot, because I am mostly a C programmer and prefer to program on Linux with multi-platform support. If you are looking into using QML I have recently had a lot of experience with that. I would be happy to answer questions. I think in the case of QT and QML the best thing to do is try things out with your own project or ask specific questions. Asking questions would especially help with QML, because it has a lot of quirky behaviours.
QT reference: http://doc.qt.io/ or http://doc.qt.io/qt-5/index.html
If you happen to need JNI with your Java I would be happy to answer questions about that too.