Useless interfaces

Question

Why would you ever use an interface if you are only going to have one implementation of it?

Answer 1

It bothers me too when people make an Interface, an Abstract, and an actual implementation for every class, even if there will never be more than one, and the 3 files are all nearly empty.

However, big uses would be:

1. Future expansion / enhancements placeholder

2. Easy to implement Inversion of Control / Dependency Injection

3. Easy to implement mocks for unit testing.

*Edit:

I noticed you have Spring in your tags too. If using Spring, then #2 above is probably the biggie. Its easier to design classes that expect interfaces, and you can swap out actual implementations of the interface in the Spring configuration (Dependency Injection).