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Not let's be clear of you I don't want to be from Effective Java which says prefer class hierarchies

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to tactlessness.

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We know that this is a class hierarchy for present bookmarks.

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It has four classes with one super class and three subprocess.

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Just note that in addition Weiss the abstractness bookmark is specifically marked with the keyword abstract

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right about the class.

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OK an item says that we need to use such a class hierarchy instead of a bag class.

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So let's look at what a that class as on why we shouldn't use it.

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Here is an example of a stack class and with this design instead of four classes we know how only one

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class which is the bookmark us so no separate classes to represent weblink movie and book bookmark itself

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.

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Below represent these entities and for the sake of brevity some of the information has been left out

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only a few fees related to movie unbroke are included and that's sufficient to understand what it across

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.

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Basically the way it works is it inkers instance variables are relevant to both movie and book.

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When a bookmark object is created and if it is off by book then only feels relevant to book or initialized

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.

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For example as much as a book then co-anchored would invoke the first constructor and pass the title

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of the book as well as the authors of the book.

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So only that relevant instance variables Baida and authors get initialized while the other movie related

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variables like directors and cast are just ignored.

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Also there is this field called bookmark type which will be initialized the w book.

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So the more course information about the type of book mark and be referred to it as the backfield as

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it is kind of tagging the bookmark object as either a book or a movie.

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Nice bookmark is a movie then client code would invoke the second constructor and would pass the title

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of the movie as well as the directors of the movie along with the cast permission.

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Not only instance variables are relevant to movie or initialized Well author's field is left untouched

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and marked by variable will now be initialized with the value Maulvi just know that.

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In this class if multiple values are to be assigned to variables like auteurs detectors on cast then

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those values would be concatenated in some way to form a string.

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Example would be a comma operator list.

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It is fine for this discussion but in practice you should avoid using such kind of concatenated values

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.

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You would instead use Arrius or other data structures like orderless.

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Next if you look at ease keep friendly eligible motet you can notice that it includes separate logic

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depending on the type of book 1.

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So basically the item in effective Java says that we shouldn't design such a class due to several reasons

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that are evident here in this illustration.

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One reason is it is boss that is you have multiple constructors too many feels on too much of logic

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and one single method.

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Second reason is it is error prone as you are trying to represent too many things in the same class

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.

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Recall the classes should be designed with single responsibility principle which was discussed in coding

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conventions.

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Listen Finally it is inefficient.

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That is English memory footprint as any object will include some irrelevant feats.

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For example when creating a book object we need to unnecessarily look at space for the variables directors

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and uncashed.

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So that's not good.

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So basically that class is not good and sometimes out of ignorance.

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Engineers might write such process.

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We don't know that we shouldn't do this.

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Since Julys an object oriented language.

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We should try to use it class hierarchy like the way we have done for the bookmark process.

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Now in case you encounter that class you may want refactoring.

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And let's see how that can be done.

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Let's just refactor our Mark back to us.

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Make the backglass an abstract class.

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Also make every method whose behavior depends on the track field as an abstract method.

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In our case we have only one such method.

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If there are any methods that do not depend on the fact free let them remain as concrete methods in

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our example.

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We do not have such methods.

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Similarly if there are any fields that are used by all types of that class then those fields should

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be in this abstract class.

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And in our example it would be the ideal variable as title applies to both movies as well as book.

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Next we find a concrete subclass for each type of duck back to us.

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The concrete classes should override the abstract methods in the superclass thus providing a specific

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implementation.

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Finally most any subclass specific variables from superclass include a corresponding subclass.

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So the more authors to book class and drippers on cost to movie class with this design there are no

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more irrelevant things and the code is simple and clear.

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Another advantage is that the code can lead to better design and more flexibility.

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For example let's say we want to create an ebook as a special type of book.

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Then we can simply create a new class called ebook which would extend the bookless under present in

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the same with a class could lead to more confusing or so as we know inheritance Palmet better design

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and we are also not touching our own code and we are simply introducing this new class.

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So that's the flexibility aspect.

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So in summary you should prefer using a class hierarchy and in case you want to attack class C if you

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want to refactor it into a class hierarchy.

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Thank you