The project cannot be built until the build path errors are resolved.

1-Right CLick on your project folder, Choose Build Path > Configure Build Path
2-Select Libraries Tab and delete any arbitrary library present there.
3-Click on Add Library option, Select JRE System Library and click Next.
4-Choose last Radiobutton option Workspace default JRE and click Finish.
5-press f5 for refresh.
6-run ur program .

Have you tried using Project > Clean... from the menu? This will force a new build on the selected projects in Eclipse.