How to up load java file when you post??

How to up load java file when you post??instead of long copy code in on post?When I tried to use Attachments,somehow it says invalid file? I upload my text file that ends with .java

Don't upload it as an attachment. And don't copy long segments of code either. Instead, put together an SSCCE that demonstrates the problem in as few lines as possible. Posting too much code will cause people to skip over your post. We simply don't have time to read through hundreds (or even dozens) of lines of code. The burden is on you to make it easy for us to help you. And half the time, you'll figure out your own problem in the process of creating the SSCCE.