Q:

Eclipse/Java newbie: package not found now, but it was yesterday

I'll be involved in Java development for the first time.  The application will be built on an set of Eclipse plug-ins and packages called UDOP.  This set comes with tutorials, which I'm going through.
Yesterday things were fine, and in my example code I was able to import a certain package, and extend one of the classes it contains.  When I started Eclipse today, a package in an import statement has a red line under it, with a hover-message telling me that the import cannot be resolved.  The target platform is correct, and my dependencies match what the tutorial wants. I've run Update Classpath.
Details:

the import statement is
import com.tdkc.udop.extensionpoints.views.ViewPartBase;

The red squiggly line is under com.tdkc.udop.extensionpoints.   
com.tdkc.udop is set as a dependency.  
The Target Platform is .../UDOP_SDK, 
and under that I can find the file .../UDOP_SDK/plugins/com.tdkc.udop_1.7.0/com/tdkc/udop/extensionpoints/views/ViewPartBase.class
com.tdkc.udop appears in the Package Explorer, and from within the P.E. I can navigate down and see ViewBasePart.class.

Although I'm new at Eclipse and Java, I do have moderate experience with c++ and Objective-C and Cocoa.
What are possible causes for an import not to be found?  What should I check?  Thanks.

A:

If nothing has changed since the last time everything was peachy, it might just be a matter of doing a Project -> Clean and then selecting Clean all projects like so:

Finally, click on OK and Eclipse should clean out all the projects and build everything.

