Java Programming

Hello to all, basically this one is a simple question. In many books I read I usually find code something as the following:

public IList getData(){

	IList data = new ArrayList();

	// � retrieving data.

	return data;
}

Now my question is, why do they return the interface rather then the implementation of an object? Is there maybe some performance gain? Or it is just a coding style?

Thanks and Regards,
Sim085