Design Problem

You are to design a program that will be given the name of a file containing a text document and output an alphabetical index that shows the places in the document where each word may be found. List the words as they appeared alphabetically, together with the line number of the lines on which they appeared. For example, given the input text, 
  in the midst of those words he was trying to say,
  in the midst of his laughter and glee,
  he had softly and suddenly vanished away,
  for the snark was a boojum, you see.
the output would be, 
       Word       Line Numbers
   ------------   -----
          a         4
        and         2, 3
       away         3
     boojum         4
        for         4
       glee         2
        had         3
         he         1, 3
        ...         ...
Additional requirements:
  1. Your program must treat the words "this" and "This" as the same word.
  2. Your program must ignore punctuation.

Your design should show a thorough decomposition of the problem. Be sure to encapsulate everything, even line numbers. (You shouldn't use int to represent line numbers, because line numbers can't be negative. So build a class for line numbers that doesn't allow them to be negative.) Perhaps this is a bit contrived, since you might not go to all that trouble in a real problem, but this is a design exercise, so show an exhaustive decomposition.

Draw a detailed UML class diagram of your solution, which includes all attributes and complete method signatures.