Chapter 10

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Session 19

• Chapter 10 Mechanisms for Software Reuse

2
Lab 6

• Available on the website
• This assignment asks you to design your own
  object-oriented program and build it "from
  scratch".
• Your task is to write a program that simulates a
  simple voice-mail system.
• For now, we will use a text-based interface

3
Types of Inheritance

• Specialization
• Specification
• Extension
• Combination
• Limitation
• Construction

4
Types of Inheritance

• Specialization
• Essentially no new methods in the subclass.
• Most subclass methods override inherited
  methods.
• example our game worlds
• example our BoundedBall class
• common in frameworks

5
Types of Inheritance

• Specification
• Superclass provides responsibility but no
  behavior.
• Implement an interface or extend an abstract
  class.
• example our event listeners, pinball targets
• private class MouseKeeper extends MouseAdapter
• private PinBallTarget element
• public void mousePressed ( MouseEvent e ) ...
  
• public void mouseReleased( MouseEvent e ) ...
  

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Types of Inheritance

• Extension
• Subclass uses most or all inherited methods
  as-is.
• Subclass adds new behavior and methods.
• example our MovableBall class

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Types of Inheritance

• Combination
• A class inherits from two or more classes. This
  is called multiple inheritance.
• Some OOP languages provide it (C). Some
  dont (Java, Smalltalk).
• Java does support combination through
  interfaces.
• example Budds Hole class
• class Hole extends Ball implements PinBallTarget
  
• public Hole( int x, int y ) ...
• public boolean intersects( Ball aBall ) ...
• public void hitBy ( Ball aBall ) ...

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Other Types of Inheritance

• Limitation
• The subclass primarily has methods that
  override inherited methods.
• to restrict a behavior (example Square extends
  Rectangle)
• to remove a behavior (example Set extends
  Vector)
• Limitation violates the principle of
  substitutability.

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Other Types of Inheritance

• Construction
• The subclass reuses a class because it provides
  needed functionality...
• ... but it is not necessarily true that an
  instance of the subclass is an instance of the
  superclass.
• example Javas Stack class (ouch!)
• Construction may violate the principle of
  substitutability.
• JUST DONT DO IT.

10
An Exercise

• public class Set extends Vector ...
• If limitation and construction are bad ideas,
  what should we do instead???
• Implement your own Set class that avoids the
  improper
• use of inheritance with limitation.
• Sets respond to these messages
• addElement( Object )
• removeElement( Object )
• contains( Object )
• isEmpty()
• size()

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Remember this slide?Class Interaction

• Relationships between classes
• 1. composition
• A Ball contains a Color.
• An instance of Color is a part of a Ball.
• 2. inheritance
• A MovableBall is a kind of Ball.
• A CannonWorld is a kind of Frame.
• 3. association
• A BoundedBall interacts with the Frame that
  contains it.

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An Exercise

• Inheritance is only one way to reuse a class.
  Another is composition having an instance of the
  class as an instance variable.
• So, instead of extending Vector, a Set can have a
  Vector instance variable, to which the set adds
  only elements that arent already in the set.

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Possible Solution for Set Class

• import java.util.Vector
• public class Set
• private Vector myStuff
• public Set()
• myStuff new Vector()
• // Methods provided in a bit

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Possible Methods for Set Class

• public void addElement( Object newElement )
• if ( !( myStuff.contains( newElement ) ) )
• myStuff.addElement( newElement )
• public boolean removeElement( Object newElement )
  
• return myStuff.removeElement( newElement )
• public boolean contains( Object newElement )
• return myStuff.contains( newElement )
• public boolean isEmpty()
• return myStuff.isEmpty()
• public int size()
• return myStuff.size()

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A driver class

• import Set
• public class SetDemo
• public static void main( String args )
• Set arguments new Set()
• for (int i 0 i lt args.length i)
• arguments.addElement( argsi )
• System.out.println( "Is the set empty? "
  arguments.isEmpty() )
• System.out.println( "Number of unique
  arguments " arguments.size() )

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Sample output

• javac SetDemo.java
• java SetDemo
• Is the set empty? true
• Number of unique arguments 0
• java SetDemo a b a b a b a b a b
• Is the set empty? false
• Number of unique arguments 2

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Reconsidering the Design of CannonWorld

• Why limit our shelves to shooting cannon balls?!!!

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Reconsidering the Design of CannonWorld

• But does it have to shoot only cannon balls? Of
  course not! All the CannonWorld needs is an
  object that can...
• move under its own control
• paint itself to the screen
• tell the CannonWorld where it lies relative to
  the floor and the target

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Currently, CannonWorld is hardwired to a
CannonBall

• public class CannonWorld extends Frame ...
• private CannonBall cannonBall
• ...
• public CannonWorld() ...
• cannonBall null
• ...
• ...
• protected void drawCannonBall(Graphics g)
• if ( cannonBall ! null )
• ...
• cannonBall.move()
• cannonBall.paint( g )
• ... cannonBall.y() ...
• ... cannonBall.x() ...
• cannonBall null
• ...
• ...
• private class FireButtonListener ...
• public void actionPerformed( ... ) ...

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The Current State of Affairs

• The CannonWorld class is hardwired to the
  CannonBall class in 3 ways
• It sends messages that are defined in the
  CannonBall class.
• It declares an instance variable of type
  CannonBall.
• It calls new with a request for an instance of
  class CannonBall.

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The Problem with the Current State of Affairs

• It sends messages that are defined in the
  CannonBall class.
• This limits CannonWorld to working with the
  decisions made when implementing CannonBall. If
  the CannonBall class was designed well, that
  shouldnt be a problem. But...
• It declares an instance variable of type
  CannonBall.
• This limits CannonWorld to working with an
  instance of class CannonBall, or an instance of a
  subclass of CannonBall. But...
• It calls new with a request for an instance of
  class CannonBall.
• This limits CannonWorld to working only with
  instances of class CannonBall no subclasses
  allowed!

22
Reconsidering the Design of CannonWorld

• Notice that the CannonWorld doesnt care...
• that its cannonball is affected by gravity...
  If the object moves under its own control, then
  gravity is a feature of the object flying through
  the air!
• what the x- and y-coordinates of its cannonball
  are. It cares whether the ball has hit the floor
  and the target. The messages sent by CannonWorld
  to its cannonBall are an unfortunate remnant of
  an implementation decision in the CannonBall
  class.
• Because this is true, we can generalize
  CannonWorld so that it works with more than just
  CannonBalls.

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Step 1 Make a Projectile Interface

• Write an interface named Projectile that
  specifies objects which can...
• move() according to a trajectory they know,
• paint() themselves to a Graphics context, and
• respond to boolean requests for their location
  relative to a Point.

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• import java.awt.Graphics
• import java.awt.Point
• public interface Projectile
• public void move ()
• public void paint ( Graphics g )
• public boolean northOf( Point p )
• public boolean southOf( Point p )
• public boolean eastOf ( Point p )
• public boolean westOf ( Point p )
• (Projectile.java)

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Step 1 Make a Projectile Interface

• This interface defines the essential
  responsibilities of the objects that a
  CannonWorld shoots.
• Notice that this interface requires the object to
  respond to queries about its relative location,
  rather than opening up its instance variables for
  outside inspection.
• The only methods to look at an objects instance
  variables should be its own instance methods.

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Step 2 Make CannonBall Implement the Projectile
Interface

• We would like our CannonWorld to fire an instance
  of any class that implements the Projectile
  interface, but that mean that CannonBalls must be
  Projectiles. So lets refactor the CannonBall
  class so that it also implements the Projectile
  interface
• (CannonBall.java)

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Step 3 Make CannonWorld Fire Any Projectile

• If we write our CannonWorld program to use the
  Projectile interface, then we are able to plug
  in new projectiles relatively easily.
• Programming to an abstract interface makes your
  programs easier to reuse and less likely to
  depend on implementation detail.

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Step 3 Make CannonWorld Fire Any Projectile

• In order for a CannonWorld to shoot Projectiles
  like our new CannonBalls from its cannon, we need
  to modify CannonWorld so that it uses a
  Projectile instead of a CannonBall. So
• Modify all references to the CannonBall in
  CannonWorld to be references to a Projectile.

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Step 3 Make CannonWorld Fire Any Projectile

• This causes something of a problem, because we
  cannot create an instance of an interface. So
• Move the creation of the CannonBall into an
  abstract protected helper method. The code that
  creates the CannonBall should call this method
  instead of calling new directly. This requires
  that the class be abstract, too.

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Step 3 Make CannonWorld Fire Any Projectile

• These changes are actually pretty
  straightforward...
• Change the instance variable to be a Projectile.
• Leave all null assignments and move/paint
  messages the same.
• Change requests for the CannonBalls instance
  variables (and the calculations on those values!)
  into requests to a Projectile.
• Replace the creation of the CannonBall with a
  call to the abstract method that creates
  instances.
• (ProjectileWorld)

31
We have just converted our CannonWorld program
into a framework
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The ResultA ProjectileWorld Framework

• ProjectileWorld is not just one program now. It
  is either zero programs or as many programs as
  you can imagine.
• We can no longer run the ProjectileWorld program
  by itself. In this way, it is not a program at
  all.
• But our new ProjectileWorld allows us to build a
  working program that fires the projectile of our
  choice in just two steps.

33
Using the ProjectileWorld Framework

• To implement a working ProjectileWorld, a client
  programmer need only
• Write a class that implements the Projectile
  interface.
• Write a subclass of ProjectileWorld that
  implements the makeProjectile(...) method to
  create an instance of the new Projectile class.

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Step 4 Make a CannonWorld Class

• Hey! We already have the Projectile of our
  choice the CannonBall. So we only have to do the
  second step.
• In order for a P r o j e c t i l e W o r l d to
  shoot CannonBalls from its cannon, we need to
  create a CannonWorld that uses a CannonBall
  instead of a Projectile. So
• (RevisedCannonWorld.java)

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The ProjectileWorld Framework

• We have built a ProjectileWorld framework two
  classes that define an infinite number of
  programs in terms of the abstract behaviors of
  their collaborators.
• The user of this framework is a programmer who
  instantiates the abstract behaviors in concrete
  classes. But these concrete classes inherit the
  bulk of the programs behavior from the classes
  in the framework.

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The ProjectileWorld Framework

• The ProjectileWorld framework is the same in
  principle as the Java AWT.
• The ProjectileWorld class is a lot like the
  Frame class, which allows us to build functional
  windows with relative ease.
• The Projectile interface is a lot like the
  listener interfaces that allow us to respond to
  user events with relative ease.
• The control of the application lies in
  ProjectileWorld. The programmer uses the
  framework classes with a dont call us well
  call you mentality.

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The ProjectileWorld Framework

• The makeProjectile() method is a factory method.
  It creates an object for the program, so that the
  program does not have to use the new command
  directly. (This is a common idea in frameworks,
  because we dont want our abstract class to
  commit to a class for its instance vars!)