Why is it a poor style to expose the implementation of a set in Java

Question

Possible Duplicate:
Why should the interface for a Java class be prefered?

I read in a book that It’s poor style to expose the implementation, so:

Good: Set s = new HashSet( );

Fair: HashSet s = new HashSet( );

can you explain why this is?

Answer 1

You should always program to an interface and not to an implementation. This allows loose coupling.
For instance if you replace HashSet to a TreeSet in the first case your code will not be affected. Compare such a change with the second case.
So you would be able to switch implementations without affecting client code as long as client code uses the interface instead of the concrete type.