It’s quite hateful that Eclipse always generates hidden files .project and .classpath in project folder. Sometimes you’re not awared if something goes wrong in these files.
After upgrading your Eclipse and if you found the following compile error, I’d suggest you to check .classpath in your project folder.
The project was not built since its build path is incomplete. Cannot find the class file for java.lang.Object. Fix the build path then try building this project
Most likely you would see a line like this.
<classpathentry kind="con" path="org.eclipse.jdt.launching.JRE_CONTAINER/ org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/j2re1.4.2_03"/>
The stupid Eclipse appended this for no reason. Just simply remove it to make it work again. 
/org.eclipse.jdt.internal.debug.ui.launcher.StandardVMType/j2re1.4.2_xx
this is not the problem in my case. what further can i do to solve this prob
By: Deepa on September 25, 2006
at 11:12 am
Well, it *was* the problem in my case. Thank you!
By: Haukur on January 8, 2007
at 9:38 am
This fixed my problem as well. To be clear, the resulting string value for the path attribute should be: “org.eclipse.jdt.launching.JRE_CONTAINER/”
Thanks!
By: Matt on March 20, 2007
at 9:28 pm
Still problem is not solved
By: Bilkis on May 14, 2007
at 11:59 am
So, what’s your problem exactly?
By: hochit on May 15, 2007
at 4:37 am
That worked for me!
Cheers,
//mikael
By: Mikael on June 16, 2007
at 6:17 am
For anyone found it worked or not worked, please quote your Eclipse version as well. Since I haven’t updated my Eclipse long time. I also want to know if the hack can apply to newer versions. Thanks!
By: hochit on June 16, 2007
at 10:52 am
worked for me. I’m using 3.1.0 though.
By: Carlos on July 24, 2007
at 9:08 pm
Kind Status Priority Description Resource In Folder Location
Error The project was not built since its classpath is incomplete. Cannot find the class file for javax.xml.rpc.ServiceException. Fix the classpath then try rebuilding this project. iimsweb_rpt
By: Binoy on July 25, 2007
at 9:31 am
It has worked for me too.
Thank you.
(Stupid Eclipse!)
By: Tiago on August 20, 2007
at 1:21 am
Doesn’t work for me either! Using Eclipse 3.10 on Ubuntu Linux 7.10.
By: Dude on October 24, 2007
at 3:26 pm
I had the same error even though the .classpath file seemed OK. I went to project properties, Java Build Path and removed “JRE System Library” entry. Then “Replace With -> Latest from HEAD” the .classpath file. The error was gone.
By: zaxon on April 26, 2008
at 8:02 pm