Pure Java meaning just the libraries that come with Java when you download it. No libraries that someone else besides the Java developers has developed. Realistically it's a silly idea, but it can be done.
Well, it's not such a "silly idea" anymore
since Java 8, which includes the hardware accelerated multi-media library JavaFX. I only started to examine it a few weeks ago in my very limited spare-time, but my tests so far are very satisfying.
The Fx library does provide a simple to use but solid all-around API with an accelerated and combined 2D & 3D scene-graph; the 3D part uses an adapted version of Java3D. Since Fx and its scene-graph is for general purposes -- UIs, diagrams, etc. but also casual games --, it is for sure not as efficient as a dedicated game engine. But it works well on all platforms and out of the box, and is easy enough to learn and use.
By the way, Fullscreen on and off now takes one to three lines of code and actually works! :-)
Java 8 final will be out in ~mid of this March 2014, and we already have a solid Release Candidate on Java.net
. So it can be used already now. It runs on all major platforms, also on ARM Linux powered devices.
I always advice people to use those libraries and tools which fits best for them. I think with Java 8, pure-Java is also possible for casual games, finally. As an old pure-Java fan I really do love that.