session 19 chapter 10 – mechanisms for software reuse
TRANSCRIPT
Session 19
Chapter 10 – Mechanisms for Software Reuse
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
Types of Inheritance
• Specialization
• Specification
• Extension
• Combination
• Limitation
• Construction
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
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 ) { ... }
}
Types of Inheritance
Extension• Subclass uses most or all inherited methods
as-is.
• Subclass adds new behavior and methods.
• example: our MovableBall class
Types of InheritanceCombination
• A class inherits from two or more classes. This is called multiple inheritance.
• Some OOP languages provide it (C++). Some don’t (Java, Smalltalk).• Java does support combination through interfaces.
• example: Budd’s Hole class
class Hole extends Ball implements PinBallTarget {public Hole( int x, int y ) { ... }public boolean intersects( Ball aBall ) { ... }public void hitBy ( Ball aBall ) { ... }
}
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.
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: Java’s Stack class (ouch!)• Construction may violate the principle of
substitutability.
JUST DON’T DO IT.
An Exercisepublic class Set extends Vector ...
If limitation and construction are bad ideas, what should we do instead???
Implement your own Set class that avoids the improperuse of inheritance with limitation.Sets respond to these messages:• addElement( Object )• removeElement( Object )• contains( Object )• isEmpty()• size()
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.
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 aren’t already in the set.
Possible Solution for Set Classimport java.util.Vector;
public class Set {private Vector myStuff;
public Set() {myStuff = new Vector();
}
// Methods provided in a bit}
Possible Methods for Set Classpublic 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();}
A driver classimport Set;public class SetDemo {
public static void main( String[] args ) {Set arguments = new Set();
for (int i = 0; i < args.length; i++)arguments.addElement( args[i] );
System.out.println( "Is the set empty? " + arguments.isEmpty() );
System.out.println( "Number of unique arguments: " +arguments.size() );
}}
Sample output
$ javac SetDemo.java
$ java SetDemoIs the set empty? trueNumber of unique arguments: 0
$ java SetDemo a b a b a b a b a bIs the set empty? falseNumber of unique arguments: 2
Reconsidering the Design of CannonWorld
• Why limit our shelves to shooting cannon balls?!!!
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
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( ... ) { ...cannonBall = new CannonBall (...);
...}}
...}
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.
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 shouldn’t 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!
Reconsidering the Design of CannonWorld
Notice that the CannonWorld doesn’t 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.
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.
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)
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 object’s instance variables should be its own instance methods.
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 let’s refactor the CannonBall class so that it also implements the Projectile interface:
(CannonBall.java)
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.
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.
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.
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 CannonBall’s 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)
We have just converted our CannonWorld program intoa framework
The Result:A 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.
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.
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)
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 program’s behavior from the classes in the framework.
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 “don’t call us; we’ll call you” mentality.
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 don’t want our abstract class to commit to a class for its instance vars!)