Alright. For example, if the geojson contains regions in the Country A, after it is loaded, it is supposed to zoom and be centered at those regions (without the need of hard-code).

The effect can be observed with this link - http://openlayers.org/en/v3.1.1/examples/drag-and-drop.html.

After the drag and drop of the geojson file onto the map, it automatically zoom and center at those regions. (again, no hard-code).

Technically, i think there is a need to store all the coordinates of the boundaries, and obtain the average value.

It might be tricky if a geojson contains the regions from multiple countries, say US and Asia. But usually it won't.