java: example: rely on autoboxing

AFAICT, autoboxing/unboxing was added in Java 5 (September 30, 2004).
I think we can afford to use it.  It should help us merge some Java
tests with the main ones.

However, beware that != does not unbox: it compares the object
addresses.

* examples/java/Calc.y, tests/java.at: Simplify.
* examples/java/Calc.test, tests/java.at: Improve tests.

examples/java/Calc.test

@@ -20,6 +20,13 @@ cat >input <<EOF
 EOF
 run 0 7
 
+cat >input <<EOF
+1 + 2 * 3 = 7
+(1 + 2) * 3 = 9
+EOF
+run 0 '7
+9'
+
 cat >input <<EOF
 1 + 2 * * 3
 EOF