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So with static properties out of the way.

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Let's come back to inheritance into a concept related to that and no way of adding methods to classes

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which you plan to inherit from.

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Here's our department class and as you know we're inheriting from it on the I.T. Department and the

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accounting department.

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Now we do have debt.

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Describe method here in our department class and therefore we can call this on any instance based on

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I.T. department and accounting department because these classes are based on department and they have

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access to all the properties and methods of that department base class now.

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As you learned you can override methods.

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So for example here in accounting department we could add our own describe method worship in where I

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want to do something different where it may be when I say accounting department I.D. and then access

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distort I.D. notice would not work here because ideas a private property of department and therefore

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we need to turn this into a protected one so that it's available in classes based on this class as well.

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But with that it should work.

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And if I now call describe on accounting which we can do of course let's maybe call it out.

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These two lines and instead call accounting dot describe down there we should see that new output.

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And indeed we see accounting department I.D. d.

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Now of course we could also override the described method in the I.T. department.

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But sometimes you don't just want to offer the option of overriding a method.

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Because that always exists you instead want to force the developers working with a certain class or

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extending a certain class to implement a certain method or to override a certain method.

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When would you do that.

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Well whenever you want to ensure that a certain method is a way lable in all classes based on some base

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class in this case department.

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But when you also know at the same time that the exact implementation will depend on the specific work.

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So when you can't provide a general method but you want to enforce that this method exists but the inheriting

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classes will need to provide their own implementation because you can't provide a default implementation

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in the base class.

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So in such a situation you might want to have an empty method in your base class and now force all classes

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based on that class to add and override this method and you can do so by adding the abstract keyword

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here.

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If you add abstract here you see we get an error that does is only available in an abstract class.

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So if you have one method or more with abstract in front of the method you have to add abstract here

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in front of class as well.

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And now we're good regarding that.

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But now you see I have an error here describe cannot have an implementation because it is marked as

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abstract.

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You have to remove the curly braces add a semicolon and instead add the return type does should have

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in this case white.

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So now you're just defining how this method should look like what its structure is.

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But you're not saying anything else besides this.

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Now however we get an error in the I.T. department because it does not implement the inherited abstract

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member describe which means we don't offer to describe method here and we do have to do that because

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we're based on the department class which is abstract and which has such an abstract method which means

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this method has to be implemented by any class based on this the part and class.

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So now here in I.T. department we have to add a describe method and in there we can now cancel lock

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I.T. department and then do whatever we want.

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This can be the same implementation as we have it in the accounting department but it can also be a

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different one.

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So now with that we can save this and add recompile and we get the almost same output as before but

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now we have two different implementations of the described method in our different classes based on

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department.

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Abstract can therefore be very useful if you want to.

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And Forest at all classes based on some other class share some common method or property we can all

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have abstract properties but at the same time you want to make sure that you don't have to provide concrete

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value to concrete implementation in the base class but instead the inheriting class has to do that all

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important abstract classes.

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So class is marked as abstract with this keyword can't be instantiated themselves.

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So you can't instantiate department.

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No.

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It's basically just a class that's there to be inherited from.

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So that these inheriting classes can be instantiated and inheriting classes are forced to provide concrete

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implementations for in this case to describe method following the structure you laid out here.

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So with the this keyword referring to a department instance or instance somehow based on Department

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including inherit the classes that might be in between and that it doesn't return anything.