Flex has been available on mobile devices for quite a while. You can use Flash Builder 4.6 with Flex 4.6 to target Android, IOS and PlayBook. It uses the Adobe AIR runtime, which can be embedded in the app or installed manually (Android), compiled into the app (IOS) or built into the device (PlayBook).
BTW: Flex apps are written with ActionScript, not Java, but can often talk to server-side Java.
As I am trying to decide between a tablet and similar size netbook, I was actually looking into installing a minimal development system (flex sdk, plain text editor, some sort of flash player).
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There is no MXML or AS3 compiler for Android as far as I know. Only Windows, Mac and Linux. You could use a Windows 7 tablet...
unfortunately this is not what I hoped to hear....
I was sort of fearing that there was no debug player, but the compilers come as java archives
The MXML compiler JAR is in C:\Program Files (x86)\Adobe\Adobe Flash Builder 4.6\sdks\4.6.0\lib, but I have no idea whether it can be run on Android...
For anything more than "trace" output you'll want a proper debugger. Regular Flash/Flex apps can be debugged with the standard AIR runtime on Android, but AFAIK you'll need a debugger (Flash Builder) for it to connect to.