That's briliant but from my point of view this loooks easier:
String numeroMaximo = Arrays.stream(Long.toString(number).split(""))
.sorted((a, b) -> b.compareTo(a))
.collect(Collectors.joining());

Oh man! I couldn't think it could have been done so clever like this :) Especially it avoids floating point computation and the hurdle of conversion floating point value to integer.

1. It only works if you learn the ABC song by heart!
2. You should at least place the whole alphabet into an iterable which eventually leads to other solutions!