If it is bad ,why there are some companies working on Java for embedded?
Embedded and embedded is two things.
A cellphone with an ARM processor and 2MB of memory is 'embedded' and so is a little thigamajig with a '51 and 1k of memory.
in the first case Java is not 'bad' in the second it is impossible.

you have user interface like as new cellphone and new digital camera interface,but you should use fast micro core not classic 8051.
NO, darn it, you should use a more powerful processor!!

The '51 is NOT a panacea, it is excellent for what it is intended for and blody awful for some other things.

If you want Java, use a processor capable of handling it.

Erik