'getWidth' and 'getHeight' will return the width and height of the screen whatever the size is. Taking half of those values will always get the coordinates of the center of the screen. This is presuming your cell size is one.

I know how to place an object in the centre but I mean that your vieuwing window is like 600 x 600 pixels. and that your vieuwing window stays600 x 600 but it moves over the world

Background information that may be required are: (1) the bounded state of the world (are you using 'super(int, int, int, false);' in world constructor) (2) the value of the scrolled amount ( zero or '(scrollWidth-worldWidth)/2' or something else ) If (1) unbounded and (2) zero, then the line that MikeItOrNot69 suggested will work; however, that line should not be placed in the 'act' method; instead, it should go in the world constructor or a method it calls (like 'prepare').

Using horizontal and vertical offset fields to track and control the amount of scrolling of the background image, you can code in the world class act a scrolling system that constantly returns the character to the center of the screen, moving all the scrollable actors and the background the same way (basically shifting everything in your world to keep your character in the center).

If 'creating' the world means updating the background image of the world and moving all the scrollable actors in the world, then, not really.