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    <p>Using interfaces is just generally good practice in Java. Some languages have multiple inheritance, others have duck typing, Java has interfaces. It's a key feature of the language, it lets me use </p> <ol> <li>different aspects of a class in different contexts and </li> <li>different implementations of the same contract without changing client code.</li> </ol> <p>So interfaces are a concept you should embrace in general, and then you would reap the benefits in situations like this where you could substitute your services by mock objects. </p> <p>One of the most important books about Java best practices is <a href="http://java.sun.com/docs/books/effective" rel="nofollow">Effective Java by Joshua Bloch</a>. I would highly suggest you to read it. In this context the most important part is <strong>Item 52: Refer to objects by their interfaces</strong>. Quote:</p> <blockquote> <p>More generally, you should favor the use of interfaces rather than classes to refer to objects. <strong>If appropriate interface types exist, then parameters, return values, variables, and fields should all be declared using interface types.</strong> The only time you really need to refer to an object’s class is when you’re creating it with a constructor.</p> </blockquote> <p>And if you take things even further (e.g. when using dependency injection), you aren't even calling the constructor.</p> <p>One of the key problems of switching languages is that you have to switch the way of thinking too. You can't program language x effectively while thinking in language y. You can't program C effectively without using pointers, Ruby not without duck typing and Java not without Interfaces.</p>
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    1. POHow to test Java app operating directly on external API
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    1. USSean Patrick Floyd
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    1. COWould the downvoter care to explain?
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    2. COSure (sorry for the delay)! I don't think that "Item 52" in Effective Java says that developers should *create* separate Java interfaces for every class, as you seem to imply. Note the part which says "If appropriate interface types exist ...". When they *don't* exist, it's perfectly fine to use the implicit class interface. *Separate* interfaces should not be abused.
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    3. CONo, not for every class (obviously not for entities and other value holders, also not for helper classes), but I do suggest that all service classes should be backed by interfaces (because you don't want tight coupling in services and you want the possibility to pass in mocks or proxies and frankly: because that's what interfaces were made for).
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