const is a reserved word that you can't use yet. If you put "const" in your code it will be a compiler error, no matter if you use it as a modifier such as final or as a variable name.
I'd like Sun to include const in the future though, even if it is only for arrays. Now you can specify an array reference to be final, but the contents can still be changed. The const reserved word could be used for this. It isn't at the moment though.