Following content is referenced from
http://www.scottdstrader.com/blog/ether_archives/000921.html
Opening a workspace with several projects, I was presented with the following error for two of the projects:
To resolve this error in eclipse do the following steps.
The resolution was to force a resave of the selected projects (and their .classpath files):
Open the project properties
Select Java Build Path > Libraries
Add a new, arbitrary library (to be deleted later) > OK
Wait for the workspace to refresh (or force a refresh of the project)
The error(s) will go away
Remove the dummy library
The only other references I could find were to make minor alterations of contents of the .classpath file.
Welcome to Mobile World !!! 
Jul 06, 2010 @ 20:57:39
Thanks. It helped…
Jul 06, 2010 @ 21:00:50
welcome Sampath.
Nov 10, 2010 @ 00:58:25
Hi Thanks it worked…..can i know the root cause of it….can u elaborate it.
Nov 10, 2010 @ 15:09:32
Tnanks it helped
Mar 18, 2011 @ 18:22:41
Thanks!
Jun 02, 2011 @ 04:15:03
thanks it worked for me 2.
Jun 05, 2011 @ 10:54:57
It’s much easier if you use [ project > clean ] command of eclipse
Feb 02, 2012 @ 21:01:02
Thanks, the solution above did not work, but “clean project” worked for me.
Jul 08, 2011 @ 03:02:09
clean will not fix this (in fact it makes it worse, as then the project complains all the generated files are missing and still fails to run). Only workaround is to add and remove a library.
Aug 20, 2011 @ 00:35:26
Sorry Tony…but clean does work!
My Eclipse ver. 3.7 if that means anything?
Aug 23, 2011 @ 14:46:17
Eclipse 3.7 Classic adding and removing a lib didn’t solve
doing a clean solved
Sep 07, 2011 @ 20:48:31
thank you so much..
Sep 21, 2011 @ 23:11:12
Thanks, that solved my problem!
Oct 18, 2011 @ 10:31:27
Really helps Thanks
Jan 21, 2012 @ 01:42:25
I was getting this error as well. What fixed it for me was to go into Window->Preferences->Java->Installed JREs->Execution Environments.
Click on the Execution environment that your project uses, and then make sure the there is a selected item in the list of “Compatible JREs”.
Strangely, I was NOT getting this error until recently, even though I did not have any of the “Compatible JREs” selected. It began to happen after I added an additional JRE.
Feb 28, 2012 @ 18:21:22
it helped, thank you very much!
Mar 05, 2012 @ 00:08:49
Worked for me. Would not have thought of that. Thanks for the tip!
Mar 05, 2012 @ 11:01:53
thanks
Aug 20, 2012 @ 11:57:23
For me, the error went away by using an advanced version of eclipse. No matter what i try ver 3.4.1 would still show the error. i downloaded ver 3.7 and it worked!
Aug 24, 2012 @ 07:21:43
I resolved this for a folder by right clicking on the folder.
Next, go Build Path > Configure Build Path.
Then, I removed the JRE that appeared under the Libraries tab, I was using 1.7.
I then added one that the PC had available by clicking the Add Library button.
Highlight JRE System Library and click Next.
In this JRE System Library window click the Environments… button to bring up the list of Execution Environments available.
Click on the the most recent version of Java and look for something to appear in the Compatible JREs window on the right.
If none appears then click on another version… and another… until some JRE appears in the right window. If none of this works then perhaps you need to install a JRE on your machine.
Once one does check its box and click OK, then click Finish, then OK.