In the future make sure you post a proper SSCCE. That means simple code in a single class that we can copy/compile and
test.
Your custom getColumnClass() method simply returns the class of the data to determine which renderer to use.
However the data you add to the model is
String data, not Integer data, so the String renderer will be used to render the column.
So the problem is with your data, not the method.
If the String data is indeed a number then you first need to convert it to an Integer before adding it to the model.