I am working on PhoneGap BlackberryWebWorks i have install BlackBerry WebWorks plug-in:2.5.1 and Blackberry WebWorks SDK:2.0.0. and PhoneGap 0.9.4.but while building the project
Per neo's comment, install Eclipse Java EE Developer Tools using the Eclipse -> Help -> Install new software -> Work with -> (your Eclipse version's download repository. e.g.:) Indigo.
In the filter field type, then checkbox, Eclipse Java EE Developer Tools.
Download, install, boom: no more errors. Well, not that error, anyway, YMMV.
Edit: Mayoayres added a comment below that may apply if you still get this error despite following these instructions. If you find it helpful, upvote his comment!