Simpe OS in JAVA

Hello guys. I want to create "simple OS" - if I can tell about it "Operation system". Just for improving my Java skills. I want simple source which I can compile into ISO image and run in my PC. Maximum I need from my OS to write 'Hello world' and list folders). Is is difficult? Can someone help me, I will be very thankful. I tried to look into JNode src, but there are alot of files, and it's difficult for me to understand them. Pls help me to write that simple bootable program).