Yes, that isn't defined, unfortunately

It is. Description says: Assume the second parameter will be a string holding one of two values: 'value' and 'index'.

So we don't have to consider other cases. That behaviour is undefined, we can return whatever.

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

Why do you always use Interfaces instead of classes?

Always compare Strings with equals, never with ==

Nice idea. I modified it a bit.

The last step could be simplified by leftShoes.equals(rightShoes)

Why do you use an Interface instead of a Class?

It doesn't Look like clean code, as someone could implement that Interface and create an instance of it.

Better use a final class with a private constructor if you want to prevent instantiatuon.

I know, but I find it more readable with parenthesis.

Compare strings with equals, not with "=="