The purpose of that question probably wasn't to see how well you know the String API, but to see how you react to challenges.
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus
subject: How to find the length of a String without using the length method