I love this solutions for it's learning to me more about the new things and new ways for the solutions and new concepts, I love CodeWars.
finally, i know my english is very bad "_"

just static method does all the work :)

Bonus emphasis?

Have a nice day

Somebody actually read the documentation for String, it seems.

What about test cases like this?

assertEquals("Have a nice day", SmashWords.smash(new String[] {" Have ", " a ", " nice ", " day "}));

Brilliant decision!

every time i see those 1-line solutions after for-looping everything i feel like a dummy

It's required to trim the result to avoid emtpy spaces in the beginning and the end of the resulting sentence - it is one of the requirements. I've added a test case method to check that for array [" ", "a", "b", " "] there is a result "a b", and not " a b ", but somehow my solution is marked as the one not passing anymore (was passing but the kata has changed).

Does this code still solve the kata? I might have missed something, but it throws an error about null when I tried it.

Interesting :) Thank you Zoidby

it's notation for variable number of String parameters or just an array

Why does the String... work in the argument list?

It finally appeared in Java 8 and uses StringJoiner class.

Wow, quite impressive, one line and it's done! Wonder where does the join come from? I mean, I'd never seen it before, is it new?

True, but an array can be initialized with null, then you will get NullPointerException:

String[] test = null;

and considering there was a test

@Test
public void validateNull() {
	assertNull(SmashWords.smash(null));
}

i'd go for a null check before returning.