This is an annoying Eclipse Bug which seems to bite now and then. See http://dev-answers.blogspot.de/2009/06/eclipse-build-errors-javalangobject.html for a possible solution, otherwise try the following;
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Close the project and reopen it.
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Clean the project (It will rebuild the buildpath hence reconfiguring with the JDK libraries)
OR
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Delete and Re-import the project and if necessary do the above steps again.
The better cure is to try NetBeans instead of Eclipse 🙂