You can't write a complete OS in C, much less Java. I shudder to think how slow it would be if it were coded in Java.
If you want information on programming operating systems I suggest the book MMURTL. It is very heavy on assembly language (but OS programming necessitates being heavy on asm) so you may want to brush up on that or learn it. I don't have a link for the book but perhaps Fordy might.
