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    <p> Let's take your example of a Dog and a Cat class, and let's illustrate using C#:</p> <p>Both a dog and a cat are animals, specifically, quadruped mammals (animals are waaay too general). Let us assume that you have an abstract class Mammal, for both of them:</p> <pre class="lang-cs prettyprint-override"><code>public abstract class Mammal </code></pre> <p>This base class will probably have default methods such as:</p> <ul> <li>Feed</li> <li>Mate</li> </ul> <p>All of which are behavior that have more or less the same implementation between either species. To define this you will have:</p> <pre class="lang-cs prettyprint-override"><code>public class Dog : Mammal public class Cat : Mammal </code></pre> <p>Now let's suppose there are other mammals, which we will usually see in a zoo:</p> <pre class="lang-cs prettyprint-override"><code>public class Giraffe : Mammal public class Rhinoceros : Mammal public class Hippopotamus : Mammal </code></pre> <p>This will still be valid because at the core of the functionality <code>Feed()</code> and <code>Mate()</code> will still be the same.</p> <p>However, giraffes, rhinoceros, and hippos are not exactly animals that you can make pets out of. That's where an interface will be useful:</p> <pre class="lang-cs prettyprint-override"><code>public interface IPettable { IList&lt;Trick&gt; Tricks{get; set;} void Bathe(); void Train(Trick t); } </code></pre> <p>The implementation for the above contract will not be the same between a cat and dog; putting their implementations in an abstract class to inherit will be a bad idea. </p> <p>Your Dog and Cat definitions should now look like:</p> <pre class="lang-cs prettyprint-override"><code>public class Dog : Mammal, IPettable public class Cat : Mammal, IPettable </code></pre> <p>Theoretically you can override them from a higher base class, but essentially an interface allows you to add on only the things you need into a class without the need for inheritance.</p> <p>Consequently, because you can usually only inherit from one abstract class (in most statically typed OO languages that is... exceptions include C++) but be able to implement multiple interfaces, it allows you to construct objects in a strictly <em>as required</em> basis.</p>
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    1. USJon Limjap
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    1. COI don't think it is that simple. You have slightly changed the question (requirements) so that interface makes more sense. You should always ask yourself whether you are defining a contract (interface) or a shared implementation (base class).
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      1. USDavid Pokluda
    2. COThe interface is a contract. You're only exposing the part of the contract that a service requires. If you have a 'PettingZoo', you certainly don't want to expose 'Mate'-ing to the user!
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      1. USAnthony Mastrean
    3. CO@David Touche, although I did it to better illustrate what an interface is for and what an abstract class is for vis-a-vis his understanding. A dog and a cat does not appear to be a rigid requirement!
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