I like this idea too. the code is very clean

I like the idea, I Too it and replaced array copy with Arrays.asList and subList, which should be better for Performance.

Impotent to notice here this solution isn't performance-wise.
It allocates in memory lots of intermediate arrays. That might be costly and has to be avoided.

Your solution is using a library?

Why people are voting "Best Practice" to a String concat loop?

ну ты гнида

Description says "Don't modify the input arr in your code". Arrays.sort(arr) does exactly the opposite.

It's possible to solve this problem in O(n*log(n)) time by reversing the keys and values of the second argument, and this is a O(n^2) solution.

It's my solution xD How dare you? Have a nice day

Such finesse! Being new to java myself, I didn't realize it had these tools to make this solution so succinct.

String str = "abc";

is equivalent to:

char data[] = {'a', 'b', 'c'};

(http://docs.oracle.com/javase/8/docs/api/java/lang/String.html)