How to build a mesh of the outside surface of joined objects?

I mean, if objects joined together has non manifold issue inside but their intersected outside surface can be water-tight, and we don’t need the inside detail, can we build a mesh of the outside surface in a simple way?

Have them be desperate objects. Move one to another layer. Then add a Boolean modifier to one. Set its object to the hidden one. Set the operation to union. There you go! You can apply the modifier if you like.

Thank you scotch. I know the boolean modifier, but that’s not what i want. What i really want is, if the model consists of lots of objects(so it’s hard to use boolean one by one), blender extract the outer surface in a simple way. Can that be possible?