[timclark]

Lots of Java development is carried out for internal business applications and like it or not they would typically be integrated with third parties using something like SOAP or a shared database. So in this type of application the choice of a concrete class over an interface probably has little impact on the known or unknown clients of your code.

However, as you point out, if you are presenting a Java API to your known and unknown clients you had better think carefully about which abstractions you are going to expose and interfaces are probably going to be very helpful.