If you put the cursor in "BufferedReader" above and then press Alt-O, the IDE opens the "BufferedReader" source. No surprise there, right? The key binding Alt-O is "Go to Source" and so, when you put the cursor on the name of a class, and then press Alt-O, you see the source.
However, what happens when you put the cursor on "java" or on "io" in the import statement above? Now you're not in the name of a class, but in the name of a package. Put the cursor in "java" for example and then press Alt-O again. And look... wow... a whole new explorer window appears, with the name of the package in the title bar and all the subpackages within the package displayed, which you can browse and you can open the classes and look at them inside the editor:
Pretty cool. I never knew this was possible. Makes NetBeans even more of a learning tool than I had thought it was. This is definitely worthy of being the 'keyboard shortcut of the week', with pride of place in the left sidebar of this blog.