Some Q&A:
Q. Newbie question: Why doesn't my breakpoint in gcj get resolved? / Why are there so few source files in gcj according to gdb?
gcj is just a small driver which calls other programs (like the frontend <code>jc1</code>) to do the actual work. See DebuggingGCC.
Q. How do I find out the name of a class represented by a tree_node variable?
p debug_tree (...)
See also DebuggingGCC.
Q. After changing just gcc/java/parse.y, which makefile target is the fastest way to rebuild?
make jc1