Centroid of a shape from .shp file

How can I get centroid of a shape from Shapefile? I have used polygeom from FLEX to compute centroid from a single polygon. But my Shapefile contains many polygons, so I use polybool from mapping toolbox to join the polygons into a sinlge polygon, then use polygeom. However polybool is not always reliable to join the polygons of the shape file.

Is there any other way to get centroil?

Thank you very much. Durga