I want to port an small open source AES encryption class to Android, and it would cut my work down a lot if the API of Android is as close as possible to the Java 6 API.
How much is implemented (or referenceable), as I have tried searching about this on google, and i have not yet come up with anything useful?
Have you checked the Java library reference on http://developer.android.com/reference/ ?
IMHO In general Java 6 works. The compiler does the work for you.
Re self implementing encryption: Please try using Bouncy Castle. In general, self implementation of any known crypto algorithm isn't a good idea.
In general you shouldn't have a lot of problem as long as you're not using any graphics or sound-related libraries. Android does not support swing or awt.
Also, this isn't an android limitation, but you'll probably going to be having a lot of trouble with floats, since the platforms that android usually runs on (ARM) do not have a Floating point unit, so you might need to take that into consideration