This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
There are some efforts to write an operating system (almost) entirely in Java: JOS, JNode. None of those really took off as a serious operating system.
There have also been attempts at making CPUs that could directly execute Java bytecode as their native instruction set - see Java processor in Wikipedia for a few links. Those also haven't been very succesful.