Basically, a String is an object representing a sequence of characters. One way to access particular characters within a String is by calling String's charAt(int index) method. Once you have that char, you can test whether it's a digit or a letter by calling Character.isDigit(char c) or Character.isLetter(char c).
"We're kind of on the level of crossword puzzle writers... And no one ever goes to them and gives them an award." ~Joe Strummer sscce.org