Calculate Time Difference Between Two Clocks in String Format

The other day I was looking for calculating the time difference between two clocks in String format. I could find a better solution, so I wrote a custom one. Take a look at the Java Code below:

   1:  public static String calTimeDifference(String startHour, 

       String startMin, String endHour, String endMin){

   2:   

   3:  String total = "0";     

   4:   

   5:  if (!StringUtils.isEmpty(startHour) && 

       !StringUtils.isEmpty(startMin) && 

       !StringUtils.isEmpty(endHour) && 

       !StringUtils.isEmpty(endMin){

   6:   

   7:   int stHr = Integer.parseInt(startHour) * 60 ;

   8:   

   9:   int stMn = Integer.parseInt(startMin);

  10:   

  11:   int enHr = Integer.parseInt(endHour) * 60 ;

  12:   

  13:   int enMn = Integer.parseInt(endMin);

  14:   

  15:   int startTime = stHr + stMn;

  16:   

  17:   int endTime = enHr + enMn;

  18:   

  19:    if (endTime > startTime){

  20:   

  21:     int durationHr = (endTime - startTime)/60;

  22:   

  23:     int durationMn = (endTime - startTime)%60;

  24:   

  25:     total = Integer.toString(durationHr) + " Hr and "

         + Integer.toString(durationMn) + " Min" ;

  26:   

  27:      }

  28:   

  29:     else{

  30:   

  31:      total = "Ending Time cannot be before Starting Time";

  32:   

  33:      }

  34:   

  35:    }       

  36:   

  37:   return total;

  38:   

  39:  }

The above code should work as long as you keep in mind the pre and post conditions.

Pre-Condition:
This function assumes that all four parameters are present so if even one of them is missing it will return zero.

Post-Condition:
This function will also make sure that the ending time cannot be before the starting time if it is it will return the error message.