Class Viewer comes with a list of packages, which it stores in ClassViewerConfix.xml which is the configuration file. For full functionality, you'll need to add ones you use, where I don't know how big of an issue that is. Hadn't really thought about it until typing this post. But in any event, here are the defaults for Javase:
<Names> <pkg>java.lang.reflect</pkg> <pkg>java.lang</pkg> <pkg>java.math</pkg> <pkg>java.io</pkg> <pkg>java.util.regex</pkg> <pkg>java.util</pkg> <pkg>java.awt.event</pkg> <pkg>java.awt.datatransfer</pkg> <pkg>java.awt.im</pkg> <pkg>java.awt</pkg> <pkg>java.net</pkg> <pkg>java.applet</pkg> <pkg>org.xml.sax.helpers</pkg> <pkg>org.xml.sax</pkg> <pkg>java.security</pkg> <pkg></pkg> <pkg></pkg> <pkg></pkg> </Names>
I left empty ones at the bottom to make it easy to copy and paste for new ones.
If Class Viewer doesn't know the package you can still view that class, but you have to use the full name, for example if it didn't know java.lang, you'd have to use java.lang.String, but can just use String.
There is a quirk on search in the search field which is VERY different from search from the left hand window, as it will check through methods for which you may not have packages in the configuration file, and if it reveals those you will see bits of those packages, which you can then just add. At first I was like, maybe should fix some kind of way? But shows you packages unknown to Class Viewer and is kind of cool to me as those stand out, as you see superclass things. So yeah, search in the Search box, unlike from the window will check through the superclass as well.