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.
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.