Your method selectSeatNumber() method doesn't return anything or change any instance variable values. It does change the local values of rowNo and columnNo, but then exits without any effect on the caller.
Yep, as Fred Kleinschmidt already observed, you cannot make a change to variable that is passed in by value, it has no effect once you leave the local method scope, you need to make a setter that will give you effect back into your seatPlan. Remember: Java passes by value -- C passes by reference.
Out on HF and heard nobody, but didn't call CQ? Nobody heard you either. 73 de N7GH