chars in Java are numeric types - they are 16 bit integers that are used to represent characters in unicode. So its pefectly OK to compare them with > or < operators. Unicode 'a' - 'z' are represented by the numeric values 97-122 respectively, so the comparisons follow normal alphabetic order, ie 'c' < 'd' < 'h' < 's'
Are you sure you want to compare 'd' < 'c'? If so, what about 'a', 'b' and 'e'?
Ahha! Just seen the significance of those 4 letters. stultuske had the answer: they should be an enum, although there's no need to add the number because enums have a ordinal() method that returns their position in the order in which they were declared. (And yes, you'll want another enum with 13 values.)