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Interfaces with only one implementation

When using jmock, you can mock a particular class if you install a class improvisator. I noticed that the class humidifier is in an inherited package, so I really don't want to use it (especially because it is very easy to extract the interface using refactoring tools in my IDE). I also don't like instance variables of specific classes.

Extracting the interface, however, I noticed a pattern appearing in my code base. A lot of time an interface has only one implementing a specific class. I prefer to use the interface wherever possible, but it seems that there really are all these additional files. Also, it’s a little tedious to update the interface and update the implementation every time I want to add a new class to the class.

Is it just the price you pay for the right abstraction, or is there a better approach that I haven't thought about? Should all classes implement an interface, even if the only thing in the interface is getters / setters?

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interface IRunner {
      void run();
      int doOtherThing();
}

abstract class Thing implements IRunner {

    //this is class-specific
    abstract void run();
    //this is common to all 'Things'
    int doOtherThing() { return 0; }

    //as are these properties
    public int getProp() {...}
    public void setProp(int val) {...}

}

public class Goo extends Thing {
      public void run() {
         int i = getProp() + doOtherThing();
         makeMagicGoo(i);
      }

}
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Some useful links on this topic:
https://martinfowler.com/bliki/InterfaceImplementationPair.html
https://rrees.me/2009/01/31/programming-to-interfaces-anti-pattern/

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