I think this is some kind of a bug in Eclipse. You can fix this by trying each solution written below. If one doesn't work try the next one.
- Clean the project
- Delete the project and import it once again (You might need to clean the project again)
- If above doesn't work try this link http://dev-answers.blogspot.de/2009/06/eclipse-build-errors-javalangobject.html. You basically remove JRE Library from project and add it again to the project.
Hope this helps.