Convert a column number to a column name. Java example.

In one of my classes we built a spreadsheet program using java.  One of the problems we had to figure out was how to access the columns.  In a spreadsheet the columns are referenced by letters: column A, column AAZ, etc.  Our underlying data structure consisted of array lists of array lists.  Obviously you can’t reference a cell by a letter it has to be a zero based number reference.