I mean, using this logic, every single function can be hidden behind an interface. Even the sole implementation of the interface can be hidden behind a yet another interface.
If there's just one implementation, then the interface is not necessary!
- Very broad, unspecific contract that may even obscure the methods purpose
- You cannot modify the contract without modifying the class AND vice versa
- Shrinking a contract (taking away elements) is far harder and more likely to cause breakages in other code than growing a contract
- Mocks become more cumbersome because the contract is so broad
- Changes to the concrete class cause ripple effects in code that doesn't care about the change
And this basically never happens, but you still have to carry that extra overhead of interfaces.
Java also supports private/public method visibility, and this can be used to clearly show the contract. No need for interfaces.
If you 100% sure the one implementation will never change, then I'd say you're right. But it requires future-telling.
They intend to write a test double which implements the interface. But then they don't get around to writing tests?