why cannot I alter my project with Eclipse Plug-in?

[mysunnytime]

I used eclipse plug-in to create a new program, wrote some code and it ran successfully. But when I altered the code and ran again, the result was the same as I programmed before. I opened the "generated" folder and checked the java file, found that it was't change to the new code as well.

How can I refresh the program? And if I wanna alter the java file to gain more functions, what should I do?(for I didn't find the interface to edit the generated java file)

Thanks a lot!