That’s up to Java developers
The Haiku team cannot be responsible for everything to happen. If you know about Java programming, you are in a better position than most of us to start working on this. We will provide support on the Haiku side. There was some work towards getting SWT ported, and that uses native widgets, and it led to additions to the native API, for example to get buttons with bitmaps on them.
This is how we usually try to work: by bringing new technologies to Haiku (Java, SWT, but also WebKit for example), we are trying to see how they can be implemented in a way that also benefit the main C++ BeOS compatible API, which is what we (of course) care about most.
I don’t know what happened to that early work on an SWT port, it was not far enough to be useful for running apps but it may be a nice base to work from.