GCJ now generate type assertions when compiling from bytecode, but not when compiling from source code. The source front-end needs to be updated to call add_type_assertion() when implicit type conversions occur.
All gcj front end bugs have been fixed by the gcj-eclipse branch merge. I'm mass-closing the affected PRs. If you believe one of these was closed in error, please reopen it with a note explaining why. Thanks.