You can program 3D in java using openGL.
You still have to learn openGL which is about the same in Java & C++.
Look-up the JOGL project.

And don't forget that alot (most) of Android games are made in JAVA.

You might also consider jMonkeyEngine if you're after something higher level.

3D in Java is no more complicated (actually I'd say it's easier with the library I'm recommending) and you can potentially build quite sophisticated commercial-quality stuff. jMonkeyEngine is the best out there that I know of, everything else is either abandoned, not very popular, or too low-level.

The jump from 2D to 3D is going to be some work no matter how you do it, but I'd recommend sticking to what you're comfortable with (Java)--try out jME 3. The community is huge, active, and quite helpful. Lots of documentation & examples, demos, and so on.

If you find 3D is too big of a jump yet, look up Slick 2D, which has all the same positive aspects of jME (community, ease of use, Java, etc.).

Don't mess with LWJGL directly - it's too low level (anyway, both libs I'm recommending are basically object oriented wrapper libs for LWJGL).


(Actually, never mind Google, see links in signature...)