Java ferns reproduce in two ways, by producing additional plants from the original rhizome and by producing plantlets at the ends of the leaves.
I've read that a plant under stress will produce more babies on the leaves than on the rhizome as a way to ensure reproduction but I don't know if that's really accurate. The plantlets do seem to grow faster than the mother plant, IME.
I've got so many plantlets on mine I'm planning a java fern forest for a new 6.6g bookshelf aquarium that I'll set up soon.