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.

Try it without the use of Monad!

Hint: take the invers of digitToInt and the invers of show

Interesting, thanks, we'll have to look at that with a benchmark.

I'm currently learning programming, and writing random tests is part of my practice to understand different testing techniques.

the description says

Kumite (ko͞omiˌtā) is the practice of taking techniques learned from Kata and applying them through the act of freestyle sparring.

It's all part of my learning journey. Thank you for your insights!

I've been wondering, do you think random tests are helpful in kumites? It looks like asserting the equality of two implementations, which only works when the reference is known to be definitely correct (in which case, why not use it in the first place instead of doing testing?).

Or maybe you're writing them just for practice, I don't know.

add random test

Good work!

A few notes if you're interested (ignore if not):

• unit () return type is typically omitted
• random tests don't do much when you're testing the same function on the left and right side
• the later arguments in assert_eq! are already equivalent to format!

Forked with these changes and a few more. Feel free to ask me if you have any questions.