ok,i did the change of settings as u said, but i am getting the same error. I am not getting any errors in the console,i am getting the bluej and greenfoot windows succesfully after running. Does it cause any problem for editing the Greenfoot source code for our project.

You need to let Eclipse rebuild the project before the errors will disappear. It should have prompted you to do this when you changed the compiler compliance setting, however, you can initiate it yourself by using "clean" from the Project menu. If you have correctly set the compiler compliance level for the project, then the errors should definitely go away. The eclipse errors are just from the eclipse compiler. They do not affect editing, but you might need to rebuild using the Ant scripts after making changes. However, as I mentioned, you should not see these errors if your compiler compliance level is set correctly.

Ok. Thank you,it worked. Now all are working fine. Thank you very much for your precious time and patience.