Until this moment I thought my code is good. Damn you, kayakero!

Isn't it easier and more effective to use switch?

Not unless the kata explicitly asks you to solve the problem without using this very method.

You should provide more details on why adding String in loop is a bad practice (performance issue) and a solution (StringBuilder). I don't remember this kata but a Regex compiled in a loop plus a Scanner are overkill and even worse than a String adding. A better way is to use Pattern/Matcher and parse... function from Double and Integer.

In any case this code is not a 'Best Practices', but a quick-and-dirty simple solution.

you should not add Strings in a loop

But that would really defeat the concept of doing the kata, wouldn't it?

IntStream has method avrerage(), so it should by solved by: Arrays.stream(i).average()

the last element in map is not needed )

No, so I guess there is a bug (at least)

Will pass2() reduce expressions of the form 2x3 ?

Ok, null is expected when a function is added.

I'm stuck with functions(InterpreterTest) test case.

x = 23
y = 25
z = 0
fn one => 1
input: 'fn one => 1' expected: but was:<0.0>

My code return 0.0 when a function is added (I didn't see in description what it should return). fn one => 1 looks fine for me so I return 0.0 but I guess an Exception is expected.
conflicts test case pass and there is almost the same situation E.g : fn f => 1

Can someone tell me why ? or what is expected ?

Resolved

That is the goal, however you don't need to use the Collections library to do it. It doesn't have shrinking it included, it is focused on expansion. The primary objective is to add the ability to expand like an ArrayList to an Array.