Question

My question is simple: is there any advantage of using interfaces if they are implemented by a single class ?
I always thought that interfaces are good only when there are multiple implementations of that interface.

Thanks.

Answer 1

In a word: no. The contract that an interface means can be specified directly in your only class.

If you are clear that you won’t need another implementation of the same methods in the future, you can avoid defining an interface.

Of course, the issue here is “in the future” clause. If the project is small, without a long development/upgrade period, and well defined, you can be almost sure about what will be needed in the future.

If the project is long as is probably that it will be subjected to changes, then you will have to factor in:

• probability that you finally need an interface.
• probability that you know now what methods the interface will need in the future.
• cost of doing the interface now vs. refactoring in the future.