An isogram is a word that has no repeating letters, consecutive or non-consecutive. Implement a function that determines whether a given string contains only letters, with no letter appearing more than once in the word.
- Assume the empty string is an isogram.
- Assume null is not an isogram.
- Ignore letter case.
- A String containing any character other than a letter should throw an IllegalArgumentException
Please use Test Driven Development style to show how you would prove your code works.
Examples:
- isIsogram "Dermatoglyphics" == true
- isIsogram "aba" == false
- isIsogram "moOse" == false -- ignore letter case