first of what Integrated development environment (IDE) are u using, if any, the build target I am guessing.....is where your classes and or jar files are build to, if u are requirred to use an ant build script then that script must have a build target specified in it......I personally don't know anything about ant scripts beyond running them in IDEs that support them and even then I prefer the the realtime java builder on my IDE which is
eclipse than have to mess with build scripts.... atleast until I feel the need to deploy my apps publicly any ways
have u tried compiling and running your code from the command interface on your OS?
I have googled your error though, it seems quite general in nature, so it could be anything from broken code, a missing path or variable in your development environment...in short..... yuh have to give more info