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?

touche

Always compare Strings with equals, never with ==

Deep down each char is a number. Or I could cay has int value attached.

Oh, I didn't know about the difference. Thanks for that.

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.

Can anybody explain why it works? Because after = threre is a Boolean expression?