According to the stackoverflow:
If you have references to external sources put them in a zip file:
YourProject->rightClick->Properties->Java Build Path->libraries->…, and then most notably android.jar, but other libs can be the culprit too. Expand it and and select Source attachment, and then (if it doesn’t say ‘None’) press the ‘Edit…’-button. If that points to a directory waht you should do is compress that source-directory into a zip file and make the source attachment point to that file.Apparently eclipse/adt feels the need to refresh sources on the file-system. When they’re in a zip-file it seems confident that they have not changed….
Just unattached source code is OK.