Android :: Combining Subclasses Which Have Different Fields For Efficiency?

I'm working on a Java Android game. Games here generally need to use memory pools to avoid garbage collection.Making use of subclasses is a pain when using memory pools as you need to e.g. say how many Roamer and Chaser objects you want upfront and not just how many Enemy objects you might need.I would then have an update function that checked the "type" variable and updated accordingly. This is obviously a more C-like approach. It feels hacky though because e.g. a roamer enemy will have a "target" variable it never uses. I'm unlikely to have more than a 100 enemies in memory at a time though so it really isn't a big deal memory wise. I just want some compromise between nice code and speed.Does anyone have any comments on how best to structure this? Is merging classes like this going too far? Is this ever a good idea? Should I just use a regular class tree?