$ git reset --hard HEAD~1
Unlink of file 'Hello.java' failed. Should I try again? (y/n)
This could mean that another program is using the file, which is preventing git from "moving" the file into or out of the working directory when you are attempting to change branches. I have had this happen on Windows Vista where eclipse is the program "using" the file. The file may not be actually open in eclipse but may have been opened by a process run by eclipse. In this event, try closing the file in any applications that might have used it. If that doesn't work, completely exit any applications which may have opened the file.