Apache Commons is not a standard part of the Java language

Well it is part of the library supplied by Codewars's test runner.

There's nothing bad about using external packages.

It doesn't, because the input string is thread safe and we don't use member variables.

Can you explain How can we know does the code need thread safety or not?

IMO, IMO is superior to IMHO

I agree with you, there is, IMHO, no need at all for thread safety here

Agreed, for local variables it doesn't make sense

That's true. And you could also improve your solution.
It's low-cohesive and doesn't meet with the DRY principle. Same work needed to remove glaring redundancy in order to beautify this code.

Done

Its been months since you reply but if you are still wondering when we measure how much time it takes for an algorithm to finish, we do not measure in time but in O(n), you can search the Big O notation(if u havent already) and youll see. btw if im wrong someone correct me im a newbie aswell.

It can be simplifed even more using constant space

i see everyone talking about 'O(N)' and that kind of stuff what does it all mean. i am a newbie by the way.
lol obviously.

Thank You! Glad to see in codewars someone concerns about efficiency.
It is even possible to determine the day of the week for 1 January without new Date().

P.S. Sometimes I do much stranger things...

I just learned the join method, so I decidied it would be cool to use it in this problem. Thank you for your help :]

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