Re: strange behevior of the JVM or compiler...


Thomas wrote:
Hello again :). I m writing code like this using JCreator :


class dummy{
public final static long mod = (long)2 * 3 * 5;

}
and then from the second class I call :

System.out.println(dummy.mod);
and if I do try to change the value of ' mod ' in code by changing
initialization line I STILL GET THE OLD VALUE NO MATTER HOW MANY TIMES I DO
COMPILE AND RUN. Is it the bug in JVM or in my IDE ?


Thats not a compiler Bug. Thats a compiler optimization trick. It is
called code 'inlining'. Means, as you declared the 'mod' as final, it
is considered as a constant. So, the compiler calculates the value of
mod, and replaces in place of all of its usage at compile time, (not
runtime). Note, It is not substituted in runtime, only at compile
time.

I can explain you in this way. Suppose Class A has a final qualified
constant member 'a' and Class B uses the constant A.a,

1. When you compile Both Class A & B, the compiler replaces all
occurance of 'A.a' (in class B )by its value, at compile time.
2. If you modify the class A.a 's value and just compile it, that
change will not reflect in Class B. Because, the change is made at
compile time only -- means when you recompile B, the change will
reflect.
3. You have to just edit the Class B and save it, then recompile class
B. Now you will see, the change made to A.a, taking effect in Class B.
(If you just recompile B, some times ant builds don't recompile it.
So, edit B, save, and then recompile)


--
Manivannan.Palanichamy (@) Oracle.com
http://mani.gw.googlepages.com/index.html

.

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