you must pass an empty parameter list when calling Java methods which take no parameters, e.g.
jpl_call('java.lang.System', gc, [], _)
you must accept an @(void) result when calling void Java methods, e.g. either
jpl_call('java.lang.System', gc, [], @(void))which explicitly matches the expected result, or
jpl_call('java.lang.System', gc, [], _)which uses an anonymous variable to ignore the result.