how to coonvert decimal number in to base 64 format

Hi friends,
                   My requirement is how to convet a decimal number of
length 19(19 digit number) in to base 64.

the number may be
Example :

                     -123 or
                     -123.455 or
                       123 or
                      123.345
            Like that.If any one know iv JAVA API or how to do
that,please send your valueble suggestions to me.

Thanks,
Naveen