How to sort data in table based on multiple Header Fields selected

I have to fill the JTable data with some date, time and string values. for example my table data looks like this:
"1998/12/14","15:14:38","Unicorn1","row1"
"1998/12/14","15:14:39","Unicorn2","row2"         "1998/12/14","15:14:40","Unicorn4","row3"         "1998/12/17","12:14:12","Unicorn4","row6"                
Now the Sorted Table should be in the following way:
"1998/12/17","12:14:12","Unicorn4","row6"                
"1998/12/14","15:14:40","Unicorn4","row3"                
"1998/12/14","15:14:39","Unicorn2","row2"                
"1998/12/14","15:14:38","Unicorn1","row1"
ie First Date field should be sorted, if 2 date fields are same then sort based on time. if date and time fields are same then need to be sorted on String field.

So if any one worked on this please theow some light on how to proceed. I know how to sort based on single column.
But now i need to sort a multiple columns.So what is code change in the Comparater class.

Thanks in advance.. This is urgent....