1 coordinate system for all screen sizes can it be done ?

Hey
when i want to calculate objects moving on screen , im doing it with:
getPositionX() and getPositionY()
that returns me the object position in the screen . based on this i calculate the movement .
but is it means i need to build my position calculation functions with different values for different screen sizes ?
or is there some-kind of virtual screen coordination that will save here ?