Should interfaces be placed in a separate package? [closed]

Answer 1

For any language, putting them together in the same package is fine. The important thing is what's exposed to the outside world, and how it looks from outside. Nobody's going to know or care if the implementation is in that same package or not.

Let's look at this particular instance.

If you have all public things in one package, and private things in another package that is not publicly exposed, the client of the library sees one package. If you move the private things to the package with the publicly exposed things, but do not expose them from within the package, the client sees exactly the same thing.

Thus, this has the smell of a rule with no good reason: it's making a decision based on something being publicly visible without that decision having any effect on what's publicly visible.

That said, if in any particular instance it seems like a good idea to split the interface and implementation in to separate packages, go right ahead and do that. Reasons for doing this that come to mind are that the package is huge, or you have an alternate implementation you might want to link in instead of the standard one.

Answer 2

I think it is important to note that for the OSGi framework, it is nicer that they are in different packages so that you can easily export a whole package while hiding the implementation packages.

Answer 3

Placing both the interface and the implementation is common place, and doesn't seem to be a problem.

Take for example the Java API -- most classes have both interfaces and their implementations included in the same package.

Take for example the java.util package:

It contains the interfaces such as Set, Map, List, while also having the implementations such as HashSet, HashMap and ArrayList.

Furthermore, the Javadocs are designed to work well in those conditions, as it separates the documentation into the Interfaces and Classes views when displaying the contents of the package.

Having packages only for interfaces may actually be a little bit excessive, unless there are enormous numbers of interfaces. But separating the interfaces into their own packages just for the sake of doing so sounds like bad practice.

If differentiating the name of a interface from an implementation is necessary, one could have a naming convention to make interfaces easier to identify:

• Prefix the interface name with an I. This approach is taken with the interfaces in the .NET framework. It would be fairly easy to tell that IList is an interface for a list.

• Use the -able suffix. This approach is seen often in the Java API, such as Comparable, Iterable, and Serializable to name a few.

Answer 4

One argument for putting interfaces in different packages is that it is easier to create 'api' jars that can be distributed to consumers of your product or service. It's perfectly possible to do this with interfaces and implementations together, but simpler to script if they are in different packages.

Answer 5

There are a lot of good reasons for putting interfaces in a separate package from implementation. For example you may want to use container that supports Dependency Injection to wire up the necessary implementation(s) at run time, in which case only the interface needs to be present at build time and the implementation can be supplied at run time. Alternatively, you may want to provide multiple implementations (for example using mock implementations for testing) or multiple versions of a particular implementation at run time (for example for A/B testing, etc). For these kinds of use cases it's more convenient to package interface and implementation separately.

Answer 6

I normally put Interfaces with the implementation however I can sort of see why you may want to keep them separate. Say, for instance, someone wanted to reimplement classes based upon your interfaces, they would need a jar/lib/etc with your implementation rather than just the interfaces. With them separate you could just say "Here's my implementation of that interface" and be done with it. Like I said, not what I do but I can sort of see why some might want to.