Eclipse: the import xxx cannot be resolved

It has often happened to me that Eclipse didn't find the correct reference to a class despite the fact the jar in which the class was included was already in the project classpath.

After googling for a while I found this discussion which in fact solved my problems:

http://www.brunningonline.net/simon/blog/archives/001748.html

To sum it up you can either:
- Clean the project
- Switch Workspace
- Remove all src folders except the root one.

However it is strongly recomended to read the reference above for complete understanding of the steps to be done.