Chapter 8:

Chapter 8
Improving structure with inheritance

Overview

Chapter 7 introduced the design of classes. Chapter 8 examines a fundamental object-oriented construct for reusing code: inheritance.

8 Introduction

inheritance - defines one class as an extension on another.
super (parent) class - The class that is extended with additional fields and methods. TicketMachine is the super or parent class.
sub (child) class - The class that inherits all fields and methods of the super class. BusTicketMachine is the subclass of TicketMachine.
private - Subclasses inherit but cannot access private methods or fields.

public - TicketMachine methods that are public can be called as internal methods (i.e. called on a BusTicketMachine object). Public fields are accessible in subclass.
constructor - TicketMachine constructors can be called as super followed by any normal parameters. The TicketMachine(int ticketCost) constructor is called by super(cost) in the BusTicketMachine( int cost, String bus, String stops ) constructor below.
over-riding methods - The printTicket() method is defined in both classes, the definition used is the closest class. BusTicketMachine objects call BusTicketMachine printTicket() method, TicketMachine objects call TicketMachine printTicket() method.
super - Over-riding methods as with printTicket() blocks access to inherited methods. To call a super class method, precede the method call with super.

public class TicketMachine
{
private int price;
private int balance;
private int total;
public TicketMachine(int ticketCost)
{
price = ticketCost;
balance = 0;
total = 0;
}
public void insertMoney(int amount)
{
balance += amount;
}
public void printTicket()
{
System.out.println("# " + price +
" cents.");
total += balance;
balance = 0;
}
}

public class BusTicketMachine extends TicketMachine
{
private String bus, stops;
public BusTicketMachine( int theCost, String theBus,
String theStops )
{
super( theCost );
bus = theBus;
stops = theStops;
}
public void printStops()
{
System.out.println("# Bus " + bus );
System.out.println("# Stops " + stops );
}
public void printTicket()
{
System.out.println("# Ticket for bus " + bus );
super.printTicket();
}
}

Exercise 1 - Inheritance

Suppose that we have created a BusTicketMachine by:
BusTicketMachine btm = new BusTicketMachine( 50, "Number 3", "4th St. - Mall - Airport" );

Trace the execution of the constructors.

Trace the execution of:
btm.insertMoney( 75 );

Trace the execution of:
btm.printTicket( );

8.1 The DoME example

Database of CD's and video's.

The database will store two collections, one for CD's and the other for video's.

The class fields are listed in the object diagram for each at right.

Exercise 2 - Need for Inheritance

How are the CD and Video fields the same?

How are the CD and Video fields different?

What types are each of the fields?

UML - Unified Modeling Language, a notation for designing and understanding object oriented systems.

CD and Video

Some of the data fields and methods given in UML form that are likely needed to model a collection of CD's and video's.

The figure at right lists both the fields (top) and methods (bottom) for the CD and Video class.

Note the common fields and methods. We may determine later that others may be needed but this shows the commonality between the two classes.

We would like to avoid duplication, inheritance can be used when the child classes are cohesive with the parent.

Database objects

The database holds the CD and Video collections separately as an ArrayList object for each.

The object diagram of the database coarsely illustrates the database object with four CDs and four Videos.

8.1.2 DoME source code

Exercise 3 - Need for Inheritance

How are CD and Video methods the same?

How are CD and Video fields the same?

How are the CD and Video methods different?

How are the CD and Video fields different?

public class Video
{
   private String title;
   private String director;
   private int playingTime;
   private boolean gotIt;
   private String comment;

   public Video(String theTitle, String theDirector,
                     int time)
   {
      title = theTitle;
      director = theDirector;
      playingTime = time;
      gotIt = false;
      comment = "<no comment>";
   }

   public void setComment(String comment)
   {     this.comment = comment;   }

   public String getComment()
   {      return comment;   }

   public void setOwn(boolean ownIt)
   {      gotIt = ownIt; }

   public boolean getOwn()
   {      return gotIt;   }

   public void print()
   {
      System.out.print("video: " + title +
                                " (" + playingTime + " mins)");
      if(gotIt) {
         System.out.println("*");
      } else {
         System.out.println();
      }
      System.out.println(" " + director);
      System.out.println(" " + comment);
   }
}

public class CD
{
   private String title;
   private String artist;
   private int numberOfTracks;
   private int playingTime;
   private boolean gotIt;
   private String comment;

   public CD(String theTitle, String theArtist, int tracks,
                 int time)
   {
      title = theTitle;
      artist = theArtist;
      numberOfTracks = tracks;
      playingTime = time;
      gotIt = false;
      comment = "<no comment>";
   }

   public void setComment(String comment)
   {    this.comment = comment; }

   public String getComment()
   {    return comment; }

   public void setOwn(boolean ownIt)
   {    gotIt = ownIt; }

   public boolean getOwn()
   {    return gotIt; }

   public void print()
   {
      System.out.print("CD: " + title +
                                 " (" + playingTime + " mins)");
      if(gotIt) {
         System.out.println("*");
      } else {
         System.out.println();
      }
      System.out.println(" " + artist);
      System.out.println(" tracks: " + numberOfTracks);
      System.out.println(" " + comment);
   }
}

Database class

The database values are not stored on a file so are lost when program closed.
There is no user interface except using BlueJ.

import java.util.ArrayList;
import java.util.Iterator;

public class Database
{
   private ArrayList cds;
   private ArrayList videos;

   public Database()
   {
      cds = new ArrayList();
      videos = new ArrayList();
   }

   public void addCD(CD theCD)
   {   cds.add(theCD);   }

   public void addVideo(Video theVideo)
   {   videos.add(theVideo);   }

   public void list()
   {
      // print list of CDs
      for(Iterator iter = cds.iterator(); iter.hasNext(); ) {
         CD cd = (CD)iter.next();
         cd.print();
         System.out.println(); // empty line between items
      }

      // print list of videos
      for(Iterator iter = videos.iterator(); iter.hasNext(); ) {
         Video video = (Video)iter.next();
         video.print();
         System.out.println(); // empty line between items
      }
   }
}

Exercise 3.5 - Need for Inheritance

What fields are similar?
What methods are similar

The figure at right reflects the DoME application after four CDs and four Videos have been entered into the Database object.

Exercise 4 - DoME

In BlueJ

Open project chapter08/dome-v1
Create one CD and Video objects.
Create one Database object.
Enter the CD and Video objects into the Database object.
List the database contents.
- Do any objects have comments?
Add a comment to one of the Video objects.
List the database contents.
- Can you explain the printed output by the figure at right and examining CD and Video print() methods below?

public class Video
{

   public void print()
   {
      System.out.print("video: " + title +
                                " (" + playingTime + " mins)");
      if(gotIt) {
         System.out.println("*");
      } else {
         System.out.println();
      }
      System.out.println(" " + director);
      System.out.println(" " + comment);
   }

public class CD
{

   public void print()
   {
      System.out.print("CD: " + title +
                                 " (" + playingTime + " mins)");
      if(gotIt) {
         System.out.println("*");
      } else {
         System.out.println();
      }
      System.out.println(" " + artist);
      System.out.println(" tracks: " + numberOfTracks);
      System.out.println(" " + comment);
   }
}

8.2 Using inheritance

inheritance - defines one class as an extension on another.

Item class defines those fields and methods duplicated or common to both CD and Video class.
- The Item methods are defined public, can be inherited by the modified CD and Video class, and can be called from the CD or Video class.
- The Item fields are private, are inherited but cannot be accessed from the CD or Video class.
- Three methods, getArtist and getNumberOfTracks in CD and getDirector in Video have been added.

Item class code

public class Item
{
   private String title;
   private int playingTime;
   private boolean gotIt;
   private String comment;

   public Item(String theTitle, int time)
   {
      title = theTitle;
      playingTime = time;
      gotIt = false;
      comment = "";
   }

   public void setComment(String comment)
   {    this.comment = comment; }

   public String getComment()
   {    return comment; }

   public void setOwn(boolean ownIt)
   {    gotIt = ownIt; }

   public boolean getOwn()
   {    return gotIt; }

   public void print()
   {
      System.out.print("title: " + title + " (" + playingTime + " mins)");
      if(gotIt) {
         System.out.println("*");
      } else {
         System.out.println();
      }
      System.out.println(" " + comment);
   }
}

CD class code

Before inheritance

After inheritance

public class CD
{
   private String title;
   private String artist;
   private int numberOfTracks;
   private int playingTime;
   private boolean gotIt;
   private String comment;

   public CD(String theTitle, String theArtist,
                 int tracks, int time)
   {
      title = theTitle;
      artist = theArtist;
      numberOfTracks = tracks;
      playingTime = time;
      gotIt = false;
      comment = "<no comment>";
   }

   public void setComment(String comment)
   {    this.comment = comment; }

   public String getComment()
   {    return comment; }

   public void setOwn(boolean ownIt)
   {    gotIt = ownIt; }

   public boolean getOwn()
   {    return gotIt; }

   public void print()
   {
      System.out.print("CD: " + title +
                              " (" + playingTime + " mins)");
      if(gotIt) {
         System.out.println("*");
      } else {
         System.out.println();
      }
      System.out.println(" " + artist);
      System.out.println(" tracks: " + numberOfTracks);
      System.out.println(" " + comment);
   }
}

public class CD extends Item
{
   private String artist;
   private int numberOfTracks;

   public CD(String theTitle, String theArtist,
                 int tracks, int time)
   {
      super(theTitle, time);
      artist = theArtist;
      numberOfTracks = tracks;
   }

   public String getArtist()
   {   return artist;   }

   public int getNumberOfTracks()
   {   return numberOfTracks;   }

   public void print()
   {
      System.out.println(" " + artist);
      System.out.println(" tracks: " + numberOfTracks);
   }
}

Video class code

Before inheritance

After inheritance

public class Video extends Item
{
   private String director;

   public Video(String theTitle, String theDirector,
                     int time)
   {
      super(theTitle, time);
      director = theDirector;
   }

   public String getDirector()
   {   return director;   }

   public void print()
   {
      System.out.println(" " + director);
   }

}

8.3 Inheritance hierarchies

Exercise 5 - Inheritance hierarchies

Extend the hierarchy diagram at right to include jazz, blues, R&R, country western and rap.
Extend the hierarchy diagram at right to include musicals, science fiction, soap operas, TV shows and comedy.

8.4 Inheritance in Java

Item is the parent or super class.
Video and CD class
- Child or subclasses, inheriting from Item.
- Have methods and fields of super class Item.
- Can call public Item methods as internal methods.
- Cannot access private fields even though inherited.

Item

Video

public class Item
{
private String title;
private int playingTime;
private boolean gotIt;
private String comment;

public Item(String theTitle, int time)

public void setComment(String comment)

public String getComment()

public void setOwn(boolean ownIt)

public boolean getOwn()

public void print()

public class Video extends Item
{
private String director;

public void print()
{
System.out.println(" " + director);
}

public String getDirector()
{ return director; }

}

public class CD extends Item
{
private String artist;
private int numberOfTracks;

public void print()
{
   System.out.println(" " + artist);
   System.out.println(" tracks: " +
             numberOfTracks);
}

public String getArtist()
{   return artist;   }

   public int getNumberOfTracks()
   {   return numberOfTracks;   }
}

8.4.1 Inheritance and access rights

Exercise 6 - Inheritance and access rights
In BlueJ

Open project Chapter08/dome-v2
Create a CD object.

Call a public inherited method such as setComment().

Open class Video
Remove the extends Item

What is the result of compiling Video?
Why?

Restore the extends Item

8.4.2 Inheritance and initialization

Superclass constructor - Subclass constructor must call super-class constructor as the first statement.
Constructors initialize object fields.
Subclass must call super-class constructor to initialize inherited fields.

Item	Video	CD
public class Item { private String title; private int playingTime; private boolean gotIt; private String comment; public Item(String theTitle, int time) { title = theTitle; playingTime = time; gotIt = false; comment = ""; }	public class Video extends Item { private String director; public Video(String theTitle, String theDirector, int time) { super(theTitle, time); director = theDirector; }	public class CD extends Item { private String artist; private int numberOfTracks; public CD(String theTitle, String theArtist, int tracks, int time) { super(theTitle, time); artist = theArtist; numberOfTracks = tracks; }

Exercise 7 - Inheritance and initialization
In your head

Trace the execution of:

CD mycd = new CD("Quacker", "Duck", 300, 10000);

Trace the execution of:

Video myvideo = new Video("Revenge of the Ducks", "A. Duck", 9000);

In BlueJ

Open CD class and set a breakpoint at first line of CD class constructor.
Create a CD object.

Inspect.
Step Into to step through the execution.
What constructor was executed for super()?
Are Item fields part of the CD object in the Inspector?

One	Two	Three
public class One { private String oneS; public One(String theS) { oneS = theS; } public String getOneS() { return oneS; } }	public class Two extends One { private String twoS; public Two(String theS) { super("red"); twoS = theS; } public String getTwoS() { return twoS; } }	public class Three extends Two { private String threeS; public Three(String theS) { super("blue"); threeS = theS; } public String getThreeS() { return threeS; } }

Exercise 7.1 - Inheritance and initialization
In your head

For the Java code above, draw the UML class diagram similar to at right that shows inheritance of fields and methods.

Trace the execution of:

One myOne = new myOne("white");
System.out.print( myOne.getOneS() );

Trace the execution of:

Two myTwo = new myTwo("red");
System.out.print( myTwo.getTwoS() );

Trace the execution of:

Three myThree = new myThree("blue");
System.out.print( myThree.getThreeS() );

8.5 DoME: adding other item types

reuse - Using existing classes in a new context.

Exercise 8 - Reuse
In BlueJ

Add the following constructor with no parameters to Item:

public Item () { }

Create a class Game that inherits from Item.

Game class has no fields, constructors or methods.

Create a class VideoGame that inherits from Game.

VideoGame class has no fields, constructors or methods.

Examine the class diagram.

What class or classes are inherited by VideoGame?

8.6 Advantages of inheritance (so far)

Avoids duplication of code that is inherited (e.g. CD, Video, Game and VideoGame use same code from Item).
Reuse of inherited code by multiple subclasses.
Maintenance easier since inherited code shared by subclasses, changes in parent class automatically extended to subclasses.
Extensibility from parent class to more specific subclasses.

8.7 Subtyping

Database class is simplified because now only responsible for Items, includes CDs and Videos through inheritance.
Only one ArrayList that references both CDs and Videos.
Only one addItem() method for both CDs and Videos.
Only one list() method for both CDs and Videos.
CDs and Videos both Items through inheritance.
Database need only handle Items to handle any subclass of Item.

Without Inheritance	With Inheritance

   public void addCD(CD theCD)
   {   cds.add(theCD);   }

   public void addVideo(Video theVideo)
   {   videos.add(theVideo);   }

   public void list()
   {
      // print list of CDs
      for(Iterator iter = cds.iterator(); iter.hasNext(); ) {
         CD cd = (CD)iter.next();
         cd.print();
         System.out.println(); // empty line between items
      }

      // print list of videos
      for(Iterator iter = videos.iterator(); iter.hasNext(); )
      {
         Video video = (Video) iter.next();
         video.print();
         System.out.println(); // empty line between items
      }
   }
}

import java.util.ArrayList;
import java.util.Iterator;

public class Database
{
   private ArrayList items;

   public Database()
   {
      items = new ArrayList();
   }

public void addItem(Item theItem)
{ items.add(theItem); }

   public void list()
   {
      for(Iterator iter = items.iterator(); iter.hasNext(); )
      {
         Item item = (Item) iter.next();
         item.print();
      }
   }
}

Exercise 8.2 - Subtyping
In BlueJ

Open chapter08/dome-v2.
Create Database, Video and CD objects.
Add each Item to the database.
List the database.

8.7.1 Subclasses and subtypes

Primitive type - Non-object types such as int, boolean, char, double.

Object type - Defined by classes.

Subtype - Defined by subclass hierarchy.
A CD object is of type CD, CD is a subtype of Item.
A VideoGame object is of type VideoGame.
VideoGame is a subtype of Game.
Game is a subtype of Item.

8.7.2 Subtyping and assignment

The two following definitions are compatible.

Variables and subtypes - Variables reference objects of their type or subtype. Cannot reference (i.e. be assigned) supertype objects.

Substitution - Subtype objects can be substituted wherever supertype object allowed.

Item item1 = new Item();
Game game1 = new Game();
VideoGame vg1 = new VideoGame();

Game variable game1 can reference Game or VideoGame objects (same type or subtype) but not Item.
Item variable item1 can reference Item objects and CD, Video, Game or VideoGame objects.
VideoGame variable vg1 can reference VideoGame objects but not Game or Item.

Compatible Types

Incompatible Types

item1 = game1;
game1 = vg1;
Item item2 = new Game();
Game game2 = new VideoGame();
game1 = item1;
vg1 = game1;
vg1 = item1;
VideoGame vg2 = new Game();
Game game2 = new Item();

Variables and subtypes - Variables reference objects of their type or subtype. Cannot reference (i.e. be assigned) supertype objects.

Substitution - Subtype objects can be substituted wherever supertype object allowed.

Exercise 9 - Subtyping and assignments
Which statements are true?

Game is a subclass of Item.
Game is a superclass of VideoGame.
Game is a subclass of Video.
Game is a superclass of Video.

Item item1 = new Item();
Game game1 = new Game();
VideoGame vg1 = new VideoGame();

Which are valid (using above item1, game1 and vg1 definitions)?

item1 = game1;
game1 = item1;
game1 = vg1;
vg1 = item1;
Item item2 = new Game();
Game game2 = new Item();
Video video1 = new Game();

One	Two	Three
public class One { private String oneS; public One(String theS) { oneS = theS; }	public class Two extends One { private String twoS; public Two(String theS) { super("red"); twoS = theS; }	public class Three extends Two { private String threeS; public Three(String theS) { super("blue"); threeS = theS; }

Exercise 9.1 - Subtyping and assignments

Give the inheritance hierarchy diagram for the above classes.

Which are valid using the myOne, myTwo and myThree definitions?
One myOne;
Two myTwo;
Three myThree;

myOne = myTwo;
myOne = myThree;
myThree = myTwo;
myThree = myOne;
myOne = new Two("purple");
myTwo = new One("green");
myThree = new Two("orange");

8.7.3 Subtyping and parameter passing

Passing subtypes - Actual parameters can be passed to formal parameters of the same or supertype.
Passing supertypes - Actual parameters cannot be passed to formal parameters of a subtype.

public class Database
{
    public void addItem( Item theItem ) {}
    public void addGame( Game theGame ) {}
    public void addVideoGame( VideoGame theVidoGame ) {}
}
    Database db = new Database();
    Item item1 = new Item();
    Game game1 = new Game();
    VideoGame vg1 = new VideoGame();

Compatible

Incompatible

db.addItem( item1 );
db.addItem( game1 );
db.addItem( vd1 );
db.addGame( vd1 );
db.addVideoGame( vd1 ); db.addGame( item1 );
db.addVideoGame( item1 );
db.addVideoGame( game1 );

Passing subtypes - Actual parameters can be passed to formal parameters of the same or supertype.
Passing supertypes - Actual parameters cannot be passed to formal parameters of a subtype.

Exercise 10 - Parameter subtypes
Which are valid (using above db, item1, game1 and vg1 definitions)?

db.addItem(item1);
db.addItem(game1);
db.addItem(vg1);

Which are valid (using above db, item1, game1 and vg1 definitions)?

db.addGame(item1);
db.addGame(game1);
db.addGame(vg1);
db.addVideoGame(item1);
db.addVideoGame(vg1);
db.addVideoGame(game1);

8.7.4 Polymorphic variables

Polymorphic - Many shapes.

Polymorphic method- When inherited methods are over-ridden, the type the object references determines the method executed.

Variables and subtypes - Variables reference objects of their type or subtype. Cannot reference (i.e. be assigned) supertype objects.

Substitution - Subtype objects can be substituted wherever supertype object allowed.

Classes can over-ride inherited methods (i.e. methods with the same name) and variables can reference objects of a subclass. For example, the following executes the getS() method of class Two at line 16:

Two aTwo = new Two("purple");
aTwo.getS();

But the following executes the getS() method of class Three at line 29:

Two aTwo = new Three("purple");
aTwo.getS();

The type the object referenced is Three so the Three method was executed.

One	Two	Three
public class One { private String oneS; public One(String theS) { oneS = theS; } public String getS() { return oneS; } }	public class Two extends One { private String twoS; public Two(String theS) { super("red"); twoS = theS; } public String getS() { return twoS; } }	public class Three extends Two { private String threeS; public Three(String theS) { super("blue"); threeS = theS; } public String getS() { return threeS; } }

Exercise 11a - Polymorphism

Which are valid?
Trace the execution.
Note that One, Two and Three each have a getS() method.

   One myOne     = new One("red");
   Two myTwo     = new Two("white");
   Three myThree = new Three("blue");

   System.out.print( myOne.getS() );
   System.out.print( myTwo.getS() );
   System.out.print( myThree.getS() );

Exercise 11b - Polymorphism

Which are valid?
Trace the execution.
Note that One, Two and Three each have a getS() method.

   One myOne     = new Two("red");
   Two myTwo     = new Three("white");
   Three myThree = new One("blue");

   System.out.print( myOne.getS() );
   System.out.print( myTwo.getS() );
   System.out.print( myThree.getS() );

Database list method can handle any objects of type or subtype Item.
Item, CD and Video have separate print() methods.
item.print() polymorphic because the type of item can be Item, CD or Video.
Item class print() called.

Without inheritance

With inheritance

public class Database
{

   public void list()
   {
      // print list of CDs
      for(Iterator iter = cds.iterator(); iter.hasNext(); ) {
         CD cd = (CD) iter.next();
         cd.print();
         System.out.println(); // empty line between items
      }

      // print list of videos
      for(Iterator iter = videos.iterator(); iter.hasNext(); ) {
         Video video = (Video) iter.next();
         video.print();
         System.out.println(); // empty line between items
      }
   }

public class Database
{

   public void list()
   {
      for(Iterator iter = items.iterator(); iter.hasNext(); )
      {
         Item item = (Item) iter.next();
         item.print();
      }
   }

Exercise 11.2 - Polymorphism
Which are valid? Note that Item, CD and Video each have a print() method.

    Item item1 = new Item();
    Video video1 = new Video();
    CD cd1 = new CD();
    video1.list();
    cd1.list();
    item1.list();

8.8 The Object class

Object - The top-level class. All classes inherit from Object class.

Variables and subtypes - Variables reference objects of their type or subtype. Cannot reference (i.e. be assigned) supertype objects.

Substitution - Subtype objects can be substituted wherever supertype object allowed.

Object is the superclass of all classes.
Item is a subclass of Object.
The following are equivalent:

public class Item

public class Item extends Object

Exercise 12 - Object
Are the following equivalent according to the hierarchy at right?

public class Game extends Object
public class Game extends Item

Which are valid?

public class Database
{
    public void addObject( Object theObject )
    public void addItem( Item theItem )
    Database db = new Database();
    Object object1 = new Object();
    Item item1 = new Item();

item1 = object1;
object1 = item1;
db.addObject( item1 );
db.addObject( object1 );
db.addItem( item1 );
db.addItem( object1 );

8.9 Polymorphic collections

8.9.1 Element types

Polymorphic collections can reference different types of objects. The Java collections ArrayList and HashMap are examples, able to reference String, TicketMachine, Game or other objects.

This is possible because these collections are defined to reference Object types. From our rules repeated below, we know that a variable of type Object can reference an object any subtype.

ArrayList signatures are given below for the add() and get() methods. Because Object is the super-type of all types, an ArrayList element can reference any other type.

Variables and subtypes - Variables reference objects of their type or subtype. Cannot reference (i.e. be assigned) supertype objects.
Substitution - Subtype objects can be substituted wherever supertype object allowed.
Passing subtypes - Actual parameters can be passed to formal parameters of the same or supertype.
Passing supertypes - Actual parameters can not be passed to formal parameters of a subtype.

public class ArrayList
{
    public void add( Object theObject )
    public Object get( int index)
public class HashMap
{
    public void put( Object key, Object theObject )
    public Object get( Object key )

8.9.2 Casting revisited

Now we can understand the need for casting.

The ArrayList method get() returns a reference to an Object. Object is the superclass so cannot be assigned to a subtypes such as Item, TicketMachine, etc. It must be cast to the correct type for the assignment.

Compile errors - The compiler can detect errors where a supertype is assigned to a subtype variable in // 1 below.

Runtime errors - The compiler can not detect errors where a subtype is assigned to the incorrect type variable as in // 3 below. However, at runtime // 3 will cause the program to fail.

Why can't the compiler catch the error in // 3? Because at runtime the ArrayList element 1 could be a CD, Video or any other subtype of Object. It depends upon what we have added to the ArrayList.

ArrayList aL = new ArrayList();

aL.add( new CD() );
aL.add( new Video() );

CD cd1 = aL.get( 0 );                    // 1. Compile error, CD = Object
Video vd1 = (Video) aL.get( 1 );    // 2. Compile OK

CD cd2 = (CD) aL.get( 1 );            // 3. Compile OK but runtime error!
                                                        CD = Video.

Exercise 12.1 - Polymorphism

What is the hierarchy diagram classes One, Two and Three?

ArrayList aL = new ArrayList();

One aOne = new One("red");
Two aTwo = new Two("white");
Three aThree = new Three("blue");

aL.add( aOne );
aL.add( aTwo );
aL.add( aThree );

Diagram the ArrayList aL.

Which of the following are valid syntax?

aOne = aL.get(0);
aOne = (One) aL.get(0);
aThree = (One) aL.get(2);
aThree = (Three) aL.get(2);
aThree = (Two) aL.get(1);
((Three) aL.get(2)).getS();

Which of the following give a runtime error?

aOne = (One) aL.get(0);
aThree = (Three) aL.get(2);
aThree = (Three) aL.get(1);
aTwo = (Two) aL.get(2);
((Three) aL.get(2)).getS();

One	Two	Three
public class One { private String oneS; public One(String theS) { oneS = theS; } public String getS() { return oneS; } }	public class Two extends One { private String twoS; public Two(String theS) { super("red"); twoS = theS; } public String getS() { return twoS; } }	public class Three extends One { private String threeS; public Three(String theS) { super("blue"); threeS = theS; } public String getS() { return threeS; } }

8.9.3 Wrapper classes

Wrapper classes - One common use is a class defined to encapsulate a primitive type for use as an object.

You may have noticed that primitive types such as int, boolean, etc. are not used in collections such as ArrayList, HashMap, etc. The reason being that collections can only reference Objects.

Java already defines wrapper classes for all primitives. For example:

Integer for int
Double for double

The wrapper classes Integer and Double inherits from Object so that Integer or Double objects can be treated as normal objects while holding the value of the primitive type.

ArrayList aL = new ArrayList();
Integer integer1 = new Integer( 5 );

aL.add( integer1 );
aL.add( new Integer( 3 ) );

Integer integer2 = (Integer) aL.get( 0 );
int int1 = integer2.intValue();

int int2 = ((Integer).get( 1 )).intValue();

Exercise 13 - Wrappers

What is the value of int1 and int2?

Chapter 8
Improving structure with inheritance

Overview

8 Introduction

8.1 The DoME example

8.1.2 DoME source code

Database class

8.2 Using inheritance

8.3 Inheritance hierarchies

8.4 Inheritance in Java

8.4.1 Inheritance and access rights

8.4.2 Inheritance and initialization

8.5 DoME: adding other item types

8.6 Advantages of inheritance (so far)

8.7 Subtyping

8.7.1 Subclasses and subtypes

8.7.2 Subtyping and assignment

8.7.3 Subtyping and parameter passing

8.7.4 Polymorphic variables

8.8 The Object class

8.9 Polymorphic collections

8.9.1 Element types

8.9.2 Casting revisited

8.9.3 Wrapper classes

8.10 The collection hierarchy

8.11 Summary

Compatible Types	Incompatible Types
item1 = game1; game1 = vg1; Item item2 = new Game(); Game game2 = new VideoGame();	game1 = item1; vg1 = game1; vg1 = item1; VideoGame vg2 = new Game(); Game game2 = new Item();

Compatible	Incompatible
db.addItem( item1 ); db.addItem( game1 ); db.addItem( vd1 ); db.addGame( vd1 ); db.addVideoGame( vd1 );	db.addGame( item1 ); db.addVideoGame( item1 ); db.addVideoGame( game1 );

Chapter 8Improving structure with inheritance

Overview

8 Introduction

8.1 The DoME example

8.1.2 DoME source code

Database class

8.2 Using inheritance

8.3 Inheritance hierarchies

8.4 Inheritance in Java

8.4.1 Inheritance and access rights

8.4.2 Inheritance and initialization

8.5 DoME: adding other item types

8.6 Advantages of inheritance (so far)

8.7 Subtyping

8.7.1 Subclasses and subtypes

8.7.2 Subtyping and assignment

8.7.3 Subtyping and parameter passing

8.7.4 Polymorphic variables

8.8 The Object class

8.9 Polymorphic collections

8.9.1 Element types

8.9.2 Casting revisited

8.9.3 Wrapper classes

8.10 The collection hierarchy

8.11 Summary

Chapter 8
Improving structure with inheritance