GolamMostafa:
Cool reading. (+).(The natural behavior of a mechanical switch/button is such that when pressed, it makes a contact for a while and then starts bouncing which lasts for about 20 ms before it makes the final sit at the closing position. If the button is connected with an interrupt pin, it has to be ensured that the interrupt-pin receives only one active pulse out of these bouncing.)
Of course the filtering of these bounces could also happen later. That is in the interrupt service routine.