[reddiric]

If there is only one implementation of an Interface, "anything implementing this Interface" and "anything of this class" are the same thing.

Coding to an interface is a good thing - but it's the concept "interface", not necessarily the language feature named "Interface" that's important. If/when you have multiple concrete objects which share the same interface, it's trivial there to switch references from the concrete type to that of the Interface (you were coding to a conceptual interface representing a consistent abstraction of a single responsibility in the beginning so your Interface is interchangeable, correct?)

[timclark]

But you understand interfaces, you have just given an explanation. The candidates I interviewed didn't explain themselves at all, I think if a language has a feature you should be able to explain when and when not to use it.

I think that all Java IDEs make it trivial to introduce an interface when it is required, so unless you are producing an API for consumption by a third party you should only introduce an interface when you have multiple implementations, or a third party can legitimately produce their own implementation, and not because you think you may have multiple implementations at sometime in the future.

Also if you have a single implementation naming it InterfaceImpl is just lazy, you almost always have more information that you can use to name it, it might be a InterfaceUsingJdbc or an InterfaceFileBased for example.

[VladRussian]

> If/when you have multiple concrete objects which share the same interface, it's trivial there to switch references from the concrete type to that of the Interface

you're a lucky person that it has been trivial for you, your teammates and all the known and unknown clients of your code and components. I can only envy your experience.

[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.