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347 lines
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Markdown
347 lines
15 KiB
Markdown
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# Inheritance Polymorphism Composition
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Zeitbedarf: 120-240 min
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Bearbeiten Sie selbständig das Dokument inheritance_and_polymorphism.docx . Sie können sich helfen lassen, aber Sie müssen die Aufgabe selber lösen und auch auf Ihrem Rechner zeigen können.
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Sie werden dazu die vorgefertigten Code-Teile brauchen, die Sie in der Datei SocialNetwork.zip finden.
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Aus didaktischen Gründen werden Sie auf eine Falschprogrammierung geführt. Lassen Sie das zu und machen Sie alles im Detail durch und zeigen Sie der Lehrperson die Resultate und die Zwischenresultate.
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Wenn Sie fertig sind, geben Sie die Aufgabe ab oder zeigen Sie sie der LP.
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## start
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**Inheritance
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& Polymorphism**
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What we will learn:
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In this session we will look at the OO concept „inheritance“.
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We will learn what polymorphism means.
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We will also learn the concepts of overloading and overwriting.
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**Contents**
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> [Wrong Programming 2](#_Toc24395775)
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> [1.1 Extending a Social-Network Simulator in the wrong way 2](#_Toc24395776)
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> [1.1.1 Exercise – Adding an Event Post 2](#_Toc24395777)
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> [1.1.2 Exercise – Extending the NewsFeed class with
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> EventPost 2](#_Toc24395778)
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> [2 Using Inheritance 4](#_Toc24395779)
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> [2.1 Analysis of the wrong solution 4](#_Toc24395780)
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> [2.2 Step-by-step to better coding 4](#_Toc24395781)
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> [2.2.1 Exercise – create a new Post class as superclass 5](#_Toc24395782)
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> [2.2.2 Exercise – Simplifying the NewsFeed Class 5](#_Toc24395783)
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> [2.3 Final product 5](#_Toc24395784)
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> [2.3.1 Simple print-out and a smarter print-out of
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> attributes 6](#_Toc24395785)
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> [2.4 Some Information on Overriding methods 6](#_Toc24395786)
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> [3 Exercises for Competences in the First Column 7](#_Toc24395787)
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> [3.1 Exercise – Understanding Symbols 7](#_Toc24395788)
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> [3.2 Exercise – New Project “Flix-Bus Switzerland” 7](#_Toc24395789)
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> [3.3 Exercise – Including inheritance 8](#_Toc24395790)
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> [3.4 Alternative Exercise – Your own example 8](#_Toc24395791)
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> [3.5 Exercise – Relations 8](#_Toc24395792)
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> [3.6 Exercise – Managing your trips 8](#_Toc24395793)
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> [3.7 Exercise – Overriding Methods 9](#_Toc24395794)
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> [3.8 Exercise –Overloading Methods 9](#_Toc24395795)
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> [3.9 Exercise – UML Classdiagram (1A) 9](#_Toc24395796)
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> [3.9.1 Exercise – Testing (3A) 9](#_Toc24395797)
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#
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Wrong Programming
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## Extending a Social-Network Simulator in the wrong way
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In order to understand the benefits of inheritance, we’re going program a social
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network simulator …. and program it in the wrong way first. We will notice how
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this approach makes maintenance and extensions more difficult and complex.
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We want a program with following classes:
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MessagePost Class for messages.
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PhotoPost Class for photos.
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NewsFeed Class has a collection of message and photo posts.
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Install the classes from the downloaded source folder.
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Each post class has a display-method to print details of the post.
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### Exercise – Adding an Event Post
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Extend the SocialNetwork by adding a new type of post:
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EventPost Class for events.
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The class has following attributes:
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private String author;
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private long timeStamp;
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private int pages;
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private int likes;
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private ArrayList\<String\> comments;
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Implement the constructor and necessary methods accordingly.
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### Exercise – Extending the NewsFeed class with EventPost
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Now we obviously have to add this new post type to our *NewsFeed* class.
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private ArrayList\<EventPost\> events;
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Further, we have to initialize this list also in the constructor. And add the
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necessary methods.
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**Analysis:**
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Obviously, this approach is very tiring and error-prone.
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List down the main problems with this approach:
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#
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Using Inheritance
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## Analysis of the wrong solution
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One main problem is the duplicated code we are adding. Most attributes in all
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three post classes are the same.
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And we have to extend the NewsFeed class, with code which is repetitive and
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cumbersome. Mistakes can happen easily while extending the code. We might even
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break existing code.
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And there’s more: if we decide to change the comment attribute from
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ArrayList\<String\> to ArrayList\<Comment\> we have to change this at several
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points in the code.
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Idea: When we extend our program, we only want to add the new classes. But we
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don’t want to change the remaining classes. For example, when we add a new post
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class, we don’t want to change the NewsFeed class.
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## Step-by-step to better coding
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Inheritance is an important concept in object-orientated programming. Classes
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can inherit from other classes. This means a class can inherit attributes and
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methods from another class.
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In a first step we want to implement a *Post* class which unifies all shared
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attributes of the several post classes. From this super class all post classes
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will inherit attributes and methods. If necessary, a post class can have its own
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special attributes.
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This is how inheritance is programmed in Java:
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### Exercise – create a new Post class as superclass
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Identify the common attributes of the post classes and add these to a new Post
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class. This class is the superclass of all other post classes.
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The new structure should look like this:
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### Exercise – Simplifying the NewsFeed Class
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Refactor the *NewsFeed* class accordingly. We want one *ArrayList* which deals
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with all posts.
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## Final product
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The **NewsFeed** class only works with the new **Post** class and does not have
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any knowledge of the subclasses.
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This will simplify any extensions we do later.
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### Simple print-out and a smarter print-out of attributes
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Make sure that your superclass *Post* has the *display()* method.
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1. We want a simple print-out of the general attributes which every post has.
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1. Now
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also include the special attributes of each extended post subclass. What
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must you do in the method of the subclass in order to combine both
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attributes (from superclass and subclass)?
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- Show your results to the teacher.
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## Some Information on Overriding methods
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When printing out attributes you have to see that the superclass and subclasses
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work together. Especially you have to make sure that the print-method of the
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superclass is overridden in the subclass. You’ll find some tips on this on
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following websites:
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<http://docs.oracle.com/javase/tutorial/java/IandI/override.html>
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<https://beginnersbook.com/2014/01/method-overriding-in-java-with-example/>
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| Notes | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
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#
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Exercises for Competences in the First Column
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Do following exercises and use the internet to research for definitions or
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examples.
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Exercises 1 – 6 will show that you have understood the basic concept of
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inheritance and that you can use overriding and overloading of methods.
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Exercises 7 and 8 show that you can draw a design in detail (with IS and HAS
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relationships) and that you can implement unit-tests.
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## Exercise – Understanding Symbols
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Look at following diagrams and use examples from our daily lives (school, work,
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etc.) to show the different relationships. Fill in the blanks:
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IS-Relation HAS-Relation
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## Exercise – New Project “Flix-Bus Switzerland”
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We want to implement a small system which simulates a national bus service in
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Switzerland. The company “Flix-Bus” offers services to national but also
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international destinations. (-\> see similar exercise “Airport” in the compendio
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book).
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Our system should have following classes:
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Class Responsibility
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> BusTerminal has all information which bus leaves from which platform
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> List of platforms, Name of Terminal
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> Platform has a number and information about bus type
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> Platform number, platform size, bus service (national or international), bus
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> type (small or large), occupied or not occupied
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Travel has all specific information about a bus trip
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Destination, departure time, arrival time, national or international
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> Bus has all information of the bus type
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> bus type (double decker, single), passenger capacity, comfort (basic or 1st
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> class)
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- Before you start coding, do a design (UML classdiagram) of the relationships
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between classes. This is part of competence 1A.
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## Exercise – Including inheritance
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Flix-bus
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wants to make sure that the bus terminal can deal with different types of buses.
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In order to do this, your system should make a distinction between the vehicles
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they use. Flix-bus uses double decker coaches for international destinations and
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a smaller, single-floored bus for national destinations.
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Be creative and implement a form of inheritance.
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Class Responsibility
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Vehicle has all basic information about the vehicle
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## Alternative Exercise – Your own example
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You can also implement your own example to show an interaction between a
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hierarchy of classes and a managing or data-pool class which can deal with these
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different types. Maybe you want to start be defining some unit test cases: How
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could you write a unit test before you have the code?
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- See also the “Airport” example in the compendio book (see OneDrive folder)
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## Exercise – Relations
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Implement a main program which instantiates the objects and creates a working
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object-hierarchy.
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Our application should allow the user to do following:
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Check times when bus leaves on platform. The user can also generate a new trip
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with a bus and the system checks what platform is available for that time.
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Implement a text-based user interface which allows the user to see the schedule.
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For example: when does the bus leave for Munich and on which platform?
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## Exercise – Managing your trips
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Make sure that the platforms are correct for the right buses. All platforms can
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be used for national buses, but only a few platforms are big enough to hold
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international buses.
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## Exercise – Overriding Methods
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One common OO feature is *overriding* methods. Use method overriding to
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implement a *print-out.*
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## Exercise –Overloading Methods
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The second common feature is *overloading* methods. Do some research on this
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topic. Then show that you have understood this feature by implementing this in
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your project.
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## Exercise – UML Classdiagram (1A)
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You did a design before you started implementation. Now draw a UML-classdiagram
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which shows the relationships of your project (in particular IS- and HAS
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relationships). Use a tool to do this.
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- Show your finished design to your teacher.
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## Exercise – Testing (3A)
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Note down essential test cases which prove that your application works. Get a
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colleague to do the testing for you.
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Based on a script by Rinaldo Lanza, BBW. Adapted by Julian Käser. Latest version
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Nov. 2019
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