The following code seems to yield different results on Java and in ObjC:

Character c = '\n';
try {
    System.out.println((int) c);
} catch (Exception e) {
    System.out.println(e);
}

Compiling with javac, this yields 10 (correctly), while j2objc throws this exception:

java.lang.ClassCastException: Cannot cast object of type java.lang.Character to JavaLangInteger

Which seems to be due to the fact that unboxing + widening casts are handled for all numeric types except Character (which does not have the intValue/longValue/doubleValue etc. conversion methods).

I'll upload a PR with an attempt to address this issue shortly.