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