static and final

[mario tan]

Hi Guys!

Can anyone explain to me what is the difference between a static and a final variable? Aren't both of them the same? I also encounter such cases wherein they are both use to declare a certain variable like for example:
static final int LENGTH = 1000.

Thanks in advance..

regards,
Mario

[Keith Lynn]

No they are not the same. A static variable is one which is associated with the class itself and not a particular instance. A final variable is one which can't be modified after it is initialized.

Making a variable static and final is the closest Java has to a constant.

[Jim Yingst]

Making a variable static doesn't make it constant at all. However if a variable does represent a compile-time constant, it might as well be static. But whether or not a variable represents a constant is completely independant of whether it's marked static. In order to be a compile-time constant, a variable must be final, and it must be a primitive type or String, and it must be initialized at the time it's declared, and using a value which is itself a compile-time constant expression.

int a = 1;  // not constant
final int b = 2;  // constant
static String c = "3";  // not constant
static final String d = "4";  // constant
final double e = loadValue(); // not constant
final long f = a; // constant
final Object g = "7"; // not constant

A local variable may also be a constant, though of course a local variable cannot ever be static. If a non-local variable is a constant, then there is really no reason not to make it static (since all instances have the same value) however it's not a requirement.

In short, being static has almost nothing to do with being constant, except they do often go together.

[Keith Lynn]

Sorry, I didn't mean to imply that being static made it a constant. I was just referring to the combination of the two identifiers makes one copy of a class variable whose value can never be changed as opposed to a final instance variable where there is a seperate copy of that variable for each instance and is constant only for that instance.

[Jim Yingst]

OK. And I should note that every time I said constant, I meant a compile-time constant, which is different than just being a variable that doesn't change. Mario probably doesn't need to worry about that anyway, and should probably skip my post for now. I just didn't want misleading information to be propagated. Mario, it's probably better to just skip any mention of "constant" until you have a better understanding of static and final, which are really completely different things.

[mario tan]

Ok. Now it's quite clear to me as to what is the difference between static and final. I just thought that both of them are the same and somewhat are closely related to each other. Anyway, thanks a lot for the reply.