Exercise 8.2

After creating a comment on the CD object it is also displayed when listing the database, even though the CD was added to the database before setting the comment.

This is because the database has a reference to the CD-object and uses this reference to get the information about the object each time it prints the list. If instead the CD-object was copied into the database the change of comment would not be reflected in the database listing.

Exercise 8.4

When the 'extends Item' is removed from the source of the Video class, then the inheritance arrow in the diagram disappears.

Exercise 8.5

Yes, it is possible to call inherited methods through the sub-menu called 'inherited from Item'

Exercise 8.6

First, the Step Into button takes us to the superclass constructor, which on the subsequent Step Into's initialises the fields: title, playingTime, gotIt and comment. Then it jumps back to the CD constructor and initialises the last two fields: artist and numberOfTracks.

Exercise 8.7

The VideoGame:

/**
 * The VideoGame class represents a video game object. Information about the 
 * video game is stored and can be retrieved.
 * 
 * @author Poul Henriksen
 * @version 2005-10-03
 */
public class VideoGame extends Item 
{
    private String platform;
    private int maxPlayers;

    /**
     * Constructor for objects of class Video
     */
    public VideoGame(String theTitle, String platform, int maxPlayers, int time)
    {
        super(theTitle, time);
        this.platform = platform;
        this.maxPlayers = maxPlayers;
    }

    /**
     * Return the platform this game runs on.
     */
    public String getPlatform()
    {
        return platform;
    }    
    
    /**
     * Return the platform this game runs on.
     */
    public int getMaxPlayers()
    {
        return maxPlayers;
    }    
}

Exercise 8.8.

One possible hierarchy:

Exercise 8.9

A touch pad and mouse are both input devices for a computer. They are very similar and they could either have a common superclass (InputDevice) or one could be a superclass of the other.

Exercise 8.10

This is quite a tricky example. There are, in fact, arguments for both: Square being a subclass of Rectangle and vice versa.

At first intuition, most people would say that Square is a subclass of Rectangle, since a square is a special case of a rectangle. So following the is-a rule, Square is the subclass.

On the other hand, we know that subclasses always have more fields than superclasses (or the same number, but not less). This is because subclasses have all the fields of their superclass, plus (potentially) more.

If we look at the Rectangle/Square example, we see that Rectangle has more fields. Square needs just one field for its size (say "width"), while Rectangle needs two ("width" and "height"). So, once we look at the fields, Rectangle should inherit from Square (it inherits one field and adds one of its own).

In practice, both solutions are leading to trouble with certain operations. For example:

- If Square is the subclass, what does it mean to call a "changeHeight" method that it might inherit from Rectangle? Do "changeHeight" and "changeWidth" then do the same, or is one disabled? What if the Rectangle class has the - quite resonable -- method "setSize(int width, int height)"?

- If Rectangle inherits from Square, can I then insert an arbitrary rectangle into my collection of squares (ArraList<Square>)?

As I said at the beginning: this is quite tricky. There is no easy solution. This just shows that inheritance is not always as obvious as one would like.

Exercise 8.11

Which of the following assignments are legal, and why?

a) Person p1 = new Student();

- This is legal because Student is a subclass of Person.

b) Person p2 = new PhDStudent();

- This is legal because PhDStudent is a subclass of Person (because it is a subclass of Student which is a subclass of Person)

c) PhDStudent phd1 = new Student();

- This is not legal, because a student is not a subclass of PhDStudent.

d) Teacher t1 = new Person();

- This is not legal because a Person is not a subclass of Teacher.

e) Student s1 = new PhDStudent;

- This is legal, because PhDStudent is a subclass of Student.

Assume that the two illegal lines above are changed to:

		PhDStudent phd1;// = new Student();
		Teacher t1;// = new Person();

f) s1 = p1

- This is not legal, because a Person is not a subclass of Student. The compiler only knows the static type of p1 which is Person - it does not know the dynamic type which is Student.

g) s1 = p2

- This is not legal, because a Person is not a subclass of Student (same arguments as in f).

h) p1 = s1

- This is legal because Student is a subclass of Person.

i) t1 = s1

- This is not legal because Student is not a subclass of Teacher.

j) s1 = phd1

- This is legal because PhDStudent is a subclass of student.

k) phd1 = s1

- This is not legal because Student is not a subclass of PhDStudent.

Exercise 8.13

Nothing has to change in the Database class when we add a new Item subclass. This is because the Database never worries about the actual subclass, but treats all objects as Items.

Exercise 8.14

From the documentation to JDK 1.4.2 the following class hierarchies can be drawn:

Exercise 8.15

Download: lab-classes8.15.zip

Exercise 8.16

An example of a class hierarchy of some of the components in a computer:

Or maybe this?

Exercise 8.17

The legal statements tells us the following:

a) m=t
This tells us that T is subclass of M

b) m=x
This tells us that X is a subclass of M

c) o=t
This tells us that T is a subclass of O. But T was also a subclass of M, and Java does not allow multiple inheritance. Therefore M must be a subclass of O or vise versa.

And the illegal statements gives us:

d) o=m
M is not a subclass of O. Hence from c) we have that O is a subclass of M.

e) o=x
X is not a subclass of O

f) x=o
O is not a subclass of X

From this information we can be sure that the following relations exist:

Exercise 8.18

See the diagram in the solution to exercise 8.14.