I've separated interfaces from their implementations by different packages, it's just add clarity, the separation is good looking and it is easier to navigate the project structure. I like it. The only point which confuses me - I've started thinking I might overdo stuff. So the question is - do you think it's a good practice to split code like that?

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    possible duplicate of Do I need to use an interface when only one class will ever implement it?
    – gnat
    Commented Jul 10, 2015 at 12:27
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    @gnat I doubt those two questions have something in common
    – Eugene
    Commented Jul 10, 2015 at 12:28
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    How are they different? Commented Jul 10, 2015 at 12:41
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    @Panzercrisis this one is about project structure, it's not like I'm asking if I need to have those interfaces, of course, I need them. The one gnat pointed to is about a creation of an interface if you have only one implementation.
    – Eugene
    Commented Jul 10, 2015 at 12:45
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    @Panzercrisis it's evident I wasn't clear with my question, I tried to rephrase it.
    – Eugene
    Commented Jul 10, 2015 at 12:50

1 Answer 1


I think it is fine, and I do it.

However I would flip it from how you are doing it, the *impl on every class would drive me insane. Instead I would name the interfaces differently (IThing as I am .Net type) and have them in an Interfaces folder. Then the concrete class (Thing) is 'normal' and would not need to live in a concrete class folder, just wherever made sense.

In some cases I then publish the interfaces folder as a Contracts package for other things that want to work with my main types.

  • I actually prefer IThing too, but you may want to consider the consistency tradeoffs. Often, I change my coding style a little bit to keep things looking similar to other programming in my current language, and it may be difficult to avoid seeing other "Foo, FooImpl"s in imported Java code.
    – Katana314
    Commented Jul 10, 2015 at 13:19

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