mirror of
https://gitlab.com/harald.mueller/aktuelle.kurse.git
synced 2024-11-24 02:31:58 +01:00
347 lines
15 KiB
Markdown
347 lines
15 KiB
Markdown
|
# 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**
|
|||
|
|
|
|||
|
|
> [Wrong Programming 2](#_Toc24395775)
|
|||
|
|
|
|||
|
|
> [1.1 Extending a Social-Network Simulator in the wrong way 2](#_Toc24395776)
|
|||
|
|
|
|||
|
|
> [1.1.1 Exercise – Adding an Event Post 2](#_Toc24395777)
|
|||
|
|
|
|||
|
|
> [1.1.2 Exercise – Extending the NewsFeed class with
|
|||
|
|
> EventPost 2](#_Toc24395778)
|
|||
|
|
|
|||
|
|
> [2 Using Inheritance 4](#_Toc24395779)
|
|||
|
|
|
|||
|
|
> [2.1 Analysis of the wrong solution 4](#_Toc24395780)
|
|||
|
|
|
|||
|
|
> [2.2 Step-by-step to better coding 4](#_Toc24395781)
|
|||
|
|
|
|||
|
|
> [2.2.1 Exercise – create a new Post class as superclass 5](#_Toc24395782)
|
|||
|
|
|
|||
|
|
> [2.2.2 Exercise – Simplifying the NewsFeed Class 5](#_Toc24395783)
|
|||
|
|
|
|||
|
|
> [2.3 Final product 5](#_Toc24395784)
|
|||
|
|
|
|||
|
|
> [2.3.1 Simple print-out and a smarter print-out of
|
|||
|
|
> attributes 6](#_Toc24395785)
|
|||
|
|
|
|||
|
|
> [2.4 Some Information on Overriding methods 6](#_Toc24395786)
|
|||
|
|
|
|||
|
|
> [3 Exercises for Competences in the First Column 7](#_Toc24395787)
|
|||
|
|
|
|||
|
|
> [3.1 Exercise – Understanding Symbols 7](#_Toc24395788)
|
|||
|
|
|
|||
|
|
> [3.2 Exercise – New Project “Flix-Bus Switzerland” 7](#_Toc24395789)
|
|||
|
|
|
|||
|
|
> [3.3 Exercise – Including inheritance 8](#_Toc24395790)
|
|||
|
|
|
|||
|
|
> [3.4 Alternative Exercise – Your own example 8](#_Toc24395791)
|
|||
|
|
|
|||
|
|
> [3.5 Exercise – Relations 8](#_Toc24395792)
|
|||
|
|
|
|||
|
|
> [3.6 Exercise – Managing your trips 8](#_Toc24395793)
|
|||
|
|
|
|||
|
|
> [3.7 Exercise – Overriding Methods 9](#_Toc24395794)
|
|||
|
|
|
|||
|
|
> [3.8 Exercise –Overloading Methods 9](#_Toc24395795)
|
|||
|
|
|
|||
|
|
> [3.9 Exercise – UML Classdiagram (1A) 9](#_Toc24395796)
|
|||
|
|
|
|||
|
|
> [3.9.1 Exercise – Testing (3A) 9](#_Toc24395797)
|
|||
|
|
|
|||
|
|
#
|
|||
|
|
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.
|
|||
|
|
|
|||
|
|
1. We want a simple print-out of the general attributes which every post has.
|
|||
|
|
|
|||
|
|
1. 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/>
|
|||
|
|
|
|||
|
|
| Notes | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|
|||
|
|
|-------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
|||
|
|
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|
|||
|
|
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|
|||
|
|
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|
|||
|
|
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|
|||
|
|
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|
|||
|
|
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|
|||
|
|
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|
|||
|
|
| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |
|
|||
|
|
|
|||
|
|
#
|
|||
|
|
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
|