1. Improving structure with inheritance
1.1

2. Main concepts to be covered
Inheritance
Subtyping
Substitution
Polymorphic variables
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

3. The DoME example "Database of Multimedia Entertainment"
stores details about CDs and videos
CD: title, artist, # tracks, playing time, got-it, comment
Video: title, director, playing time, got-it, comment
allows (later) to search for information or print lists
database to store details of all CDs and videos I know
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

4. DoME objects
one object per CD or video; each object stores details for one item.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

5. DoME classes
add the obvious methods (getters and setters); not complete here - just examples
add a print method to print out the details
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

6. DoME object model
database object will hold two collections: one for CDs, one for videos
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

7. Class diagram class diagram is simple (ArrayList not shown in BlueJ)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

8. [ ] CD source code incomplete (comments!) public class CD {
private String title;
private String artist;
private String comment;
CD(String theTitle, String theArtist) {
title = theTitle;
artist = theArtist;
comment = " "; }
void setComment(String newComment)
{ ... }
String getComment()
void print()
...
CD source code [ ]
incomplete
(comments!)
extract from CD code - nothing unusual
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

9. [ ] Video source code incomplete (comments!) public class Video {
private String title;
private String director;
private String comment;
Video(String theTitle, String theDirect) {
title = theTitle;
director = theDirect;
comment = " "; }
void setComment(String newComment)
{ ... }
String getComment()
void print()
...
Video source code [ ]
incomplete
(comments!)
extract from video code (how does it differ from CD code?)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

10. Database source code class Database { private ArrayList cds;
private ArrayList videos;
...
public void list() {
for(Iterator iter = cds.iterator(); iter.hasNext(); ) {
CD cd = (CD)iter.next();
cd.print();
System.out.println(); // empty line between items }
for(Iterator iter = videos.iterator(); iter.hasNext(); ) {
Video video = (Video)iter.next();
video.print();
Database source code
extract from database code.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

11. Critique of DoME code duplication
CD and Video classes very similar (large part are identical)
makes maintenance difficult/more work
introduces danger of bugs through incorrect maintenance
code duplication also in Database class
we note a lot of code duplication.
this is one problem with this solution (there are others)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

12. Using inheritance
solution: inheritance. make superclass with common attributes, make subclasses
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

13. Using inheritance define one superclass : Item
define subclasses for Video and CD
the superclass defines common attributes
the subclasses inherit the superclass attributes
the subclasses add own attributes
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

14. Inheritance hierarchies
inheritance hierarchies are nothing unusual. we see them all the time.
(a masters student is a students is a person...)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

15. Inheritance in Java no change here change here public class Item { ...}
change here
public class Video extends Item {
... }
syntax for inheritance: extends keyword
public class CD extends Item {
... }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

16. Superclass public class Item { private String title;
private int playingTime;
private boolean gotIt;
private String comment;
// constructors and methods omitted. }
we define common fields in superclass
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

17. Subclasses public class CD extends Item { private String artist;
private int numberOfTracks;
// constructors and methods omitted. }
we add subclass fields; inherit superclass fields (note extends keyword)
subclass objects will have all fields.
public class Video extends Item {
private String director;
// constructors and methods omitted. }
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

18. Inheritance and constructors
public class Item {
private String title;
private int playingTime;
private boolean gotIt;
private String comment;
/**
* Initialise the fields of the item. */
public Item(String theTitle, int time)
title = theTitle;
playingTime = time;
gotIt = false;
comment = ""; }
// methods omitted
Inheritance and constructors
how do we initialise the fields? superclass: nothing unusual.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

19. Inheritance and constructors
public class CD extends Item {
private String artist;
private int numberOfTracks;
/**
* Constructor for objects of class CD */
public CD(String theTitle, String theArtist,
int tracks, int time)
super(theTitle, time);
artist = theArtist;
numberOfTracks = tracks; }
// methods omitted
subclass: must call superclass constructor! Must take values for all
fields that we want to initialise.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

20. Superclass constructor call
Subclass constructors must always contain a 'super' call.
If none is written, the compiler inserts one (without parameters)
works only, if the superclass has a constructor without parameters
Must be the first statement in the subclass constructor.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

21. Adding more item types it is now much easier to add new types.
common attributes do not need to be rewritten.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

22. Deeper hierarchies
when adding new types, the hierarchy may be extended
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

23. Review (so far) Inheritance (so far) helps with:
Avoiding code duplication
Code reuse
Easier maintenance
Extendibility
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

24. New Database source code
public class Database {
private ArrayList items;
/**
* Construct an empty Database. */
public Database()
items = new ArrayList(); }
* Add an item to the database.
public void addItem(Item theItem)
items.add(theItem);
...
avoids code duplication in client!
note: code duplication in class Database removed as well!
only on field, one ArrayList creation, on add method
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

25. New Database source code
/**
* Print a list of all currently stored CDs and
* videos to the text terminal. */
public void list() {
for(Iterator iter = items.iterator(); iter.hasNext(); ) {
Item item = (Item)iter.next();
item.print();
System.out.println(); // empty line between items }
New Database source code
...and only one loop in list method.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

26. Subtyping First, we had: Now, we have: We call this method with:
public void addCD(CD theCD)
public void addVideo(Video theVideo)
Now, we have:
public void addItem(Item theItem)
We call this method with:
Video myVideo = new Video(...);
database.addItem(myVideo);
Subtyping
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

27. Subclasses and subtyping
Classes define types.
Subclasses define subtypes.
Objects of subclasses can be used where objects of supertypes are required. (This is called substitution .)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

28. Subtyping and assignment
subclass objects may be assigned to superclass variables
Vehicle v1 = new Vehicle();
Vehicle v2 = new Car();
Vehicle v3 = new Bicycle();
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

29. Subtyping and parameter passing
public class Database {
public void addItem(Item theItem)
... }
Video video = new Video(...);
CD cd = new CD(...);
database.addItem(video);
database.addItem(cd);
subclass objects may be passed to superclass parameters
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

30. Object diagram
database object will hold two collections: one for CDs, one for videos
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

31. Class diagram class diagram is simple (ArrayList not shown in BlueJ)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

32. Polymorphic variables
Object variables in Java are polymorphic. (They can hold objects of more than one type.)
They can hold objects of the declared type, or of subtypes of the declared type.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

33. The Object class All classes inherit from Object.
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

34. Polymorphic collections
All collections are polymorphic.
The elements are of type Object. public void add(Object element) public Object get(int index)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

35. Casting revisited Can assign subtype to supertype.
Cannot assign supertype to subtype! String s1 = myList.get(1); error!
Casting fixes this: String s1 = (String) myList.get(1); (only if the element really is a String!)
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

36. Wrapper classes All objects can be entered into collections...
...because collections accept elements of type Object...
...and all classes are subtypes of Object.
Great! But what about simple types?
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

37. Wrapper classes
Simple types (int, char, etc) are not objects. They must be wrapped into an object!
Wrapper classes exist for all simple types:
simple type wrapper class
int Integer
float Float
char Character
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

38. Wrapper classes wrap the int value add the wrapper
int i = 18;
Integer iwrap = new Integer(i);
myCollecton.add(iwrap);
...
Integer element = (Integer) myCollection.get(0);
int value = element.intValue()
add the wrapper
retrieve the wrapper
unwrap
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

39. Review
Inheritance allows the definition of classes as extensions of other classes.
Inheritance
avoids code duplication
allows code reuse
simplifies the code
simplifies maintenance and extending
Variables can hold subtype objects.
Subtypes can be used wherever supertype objects are expected (substitution).
Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling