controlling mob spawn rate

I am trying to control the mob spawn rate at run time. Therefore I thought I could use the methodes

    public int getMaxSpawnedInChunk() original in EntityLiving

and

    public boolean getCanSpawnHere() original in EntityAnimal

But both are not called at all. Tho new entities of my class are spawned in a new world...

Now I have the questions

a) what method and at what time will call the above methods

and b) if not with these methods how can I control the spawning rate of creature type entities?