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This pattern is very simple to employ Essentially it takes advantage of the fact that an implementation has a dependency to its interface but not vice versa This means you can put the interface and the implementation in separate packages and the implementation package has a dependency to the interface package Other packages can depend on the interface package without depending on the implementation package
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Of course, the software won't work at runtime without some implementation of the interface This can be either at compile time using a separate package that ties the two together or at configuration time using Plugin (499)
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You can place the interface in the client's package (as in the sketch) or in a third package (Figure 181) If there's only one client for the implementation, or all the clients are in the same package, then you might as well put the interface in with the client A good way of thinking about this is that the developers of the client package are responsible for defining the interface Essentially the client package indicates that it will work with any other package that implements the interface it defines If you have multiple client packages, a third interface package is better It's also better if you want to show that the interface definition isn't the responsibility of the client package developers This would be the case if the developers of the implementation were responsible for it
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Figure 181 Placing the Separated Interface in a third package
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You have to consider what language feature to use for the interface For languages that have an interface construct, such as Java and C#, the interface keyword is the obvious choice However, it may not be the best
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An abstract class can make a good interface because you can have common, but optional, implementation behavior in it
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One of the awkward things about separate interfaces is how to instantiate the implementation It usually requires knowledge of the implementation class The common approach is to use a separate factory object, where again there is a Separated Interface for the factory You still have to bind an implementation to the factory, and Plugin (499) is a good way to do this Not only does it mean there is no dependency, but it also defers the decision about implementation class to configuration time
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If you don't want to go all the way to Plugin (499), a simpler alternative is to let yet another package that knows both the interface and the implementation instantiate the right objects at application startup Any objects that use Separated Interface can either themselves be instantiated or have factories instantiated at startup
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You use Separated Interface when you need to break a dependency between two parts of the system Here are some examples:
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You've built some abstract code for common cases into a framework package that needs to call some particular application code You have some code in one layer that needs to call code in another layer that it shouldn't see, such as domain code calling a Data Mapper (165) You need to call functions developed by another development group but don't want a dependency into their APIs
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I come across many developers who have separate interfaces for every class they write I think this is excessive, especially for application development Keeping separate interfaces and implementations is extra work, especially since you often need factory classes (with interfaces and implementations) as well For applications I recommend using a separate interface only if you want to break a dependency or you want to have multiple independent implementations If you put the interface and implementation together and need to separate them later, this is a simple refactoring that can be delayed until you need to do it
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There is a degree to where the determined management of dependencies in this way can get a little silly Having only a dependency to create an object, and using the interface ever after, is usually enough The trouble comes when you want to enforce dependency rules, such as by doing a dependency check at build time Then all dependencies have to be removed For a smaller system enforcing dependency rules is less of an issue, but for bigger systems it's a very worthwhile discipline
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