Compile time constant

[Apna Apana]

Hi all
what is a compile time constant?
is it a final variable?
I am not at all clear with it.

Can anyone help me in understanding the concept clearly

[Meena R. Krishnan]

Compile time constants are static finals whose values are known at compile-time.

eg. static final int i =100;


whereas load-time constants are the ones whose values are not known at compile-time but only at runtime.

eg. static final int i = getReturnFromSomeMethod();

[Apna Apana]

Actually i have seen an example like this:


class JSC202 {
static byte m1() {final short s1 = 2; return s1;} // 1
static byte m2(final short s2) {return s2;} // 2
public static void main(String[] args) {
short s3 = 4;
System.out.print(""+m1()+m2(s3)); // 3
}}


where is error?
Ans: Compile time Error at line 2.

Please justify the answer.


Is it that any final variable is a compile time constant or a static final variable is only the compile time constant.

[Sasha Ruehmkorf]

I think a "compile time constant" is just an abbreviation for what is called "compile time constant expression" in the language specification.
The definition is found here: http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#5313
Especially note "Simple names that refer to constant variables (�4.12.4)." and the link leads to this definition: "We call a variable, of primitive type or type String, that is final and initialized with a compile-time constant expression (�15.28) a constant variable."

So, static or not, is not of interest. But the type of the variable should be a primitive or a String. Thanks for the question, as I also haven't been sure about what "compile time constant" means yet. But now I am

Regarding the code: Have you tried to compile it? I don't get an error at line2, I get an error at line3: "Type mismatch: cannot convert from short to byte"

edit:
In short, you can say "compile time constants" are final variables of primitive type that are given a literal-value at the time of declaration. Literals itself are also called "compile time constant", so:

final int i1;
i1 = 4;
final int i2 = 4;

If I understood it right the variable i2 is a compile-time-constant, whereas i1 is not. That means i1 cannot be used as a case in a switch-statement, but i2 can.
[ May 17, 2007: Message edited by: Sasha Ruehmkorf ]