1. Session 15 Chapter 8: Understanding Inheritance

2. Lab Exercise Solution Recall that we needed to know if a mousePress occurred on a target. We tried to leverage the intersects method of PinBallTarget interface by creating a temporary ball at the point of the mouse press: Ball tempBall = new PinBall( e.getX(), e.getY() ); for ( int j = 0; j < targets.size(); j++ ) { PinBallTarget target=(PinBallTarget) targets.elementAt(j); if ( target.intersects( tempBall ) ) { startPressed = 1; element = target; } // end if } // end for

3. Question If you had to pick one word that identifies the key concepts of this course so far, what would it be? Why?

4. My Answer If you had to pick one word that identifies the key concepts of this course so far, what would it be? Why? –I think I might pick “Inheritance”

5. Inheritance What? A mechanism for reusing code in an existing class. A mechanism for organizing kinds of objects that have the same implementation. How? Create a class that extends another class. Why? Who wants to rewrite code?? Reuse provides: reliability through continual testing shorter development time ability to build frameworks (don’t call us...) You can quickly build an application for demonstration purposes.

6. Exercise On Page 132, Budd describes how we might implement the class Set as a subclass of class Vector. A Set lets you add an element to it only if the Set doesn’t already contain that element. What messages should a set respond to?

7. Another Exercise On Page 132, Budd describes how we might implement the class Set as a subclass of class Vector. A Set lets you add an element to it only if the Set doesn’t already contain that element. –addElement( Object ) –removeElement( Object ) –contains( Object ) –isEmpty() –size() If ‘Set extends Vector’, which of these are already in existence? Which are correct? http://java.sun.com/j2se/1.4.2/docs/api/java/util/Vector.html

8. Another Exercise On Page 132, Budd describes how we might implement the class Set as a subclass of class Vector. A Set lets you add an element to it only if the Set doesn’t already contain that element. –addElement( Object ) –removeElement( Object ) –contains( Object ) –isEmpty() –size() Write the addElement method that we need to complete the Set class.

9. Another Exercise On Page 132, Budd describes how we might implement the class Set as a subclass of class Vector. A Set lets you add an element to it only if the Set doesn’t already contain that element. –addElement( Object ) –removeElement( Object ) –contains( Object ) –isEmpty() –size() If ‘Set extends Vector’ What other problem(s) do we have?

10. A possible solution import java.util.Vector; public class Set extends Vector { public Set() { super(); } public void addElement( Object object ) { if ( !contains( object )) super.addElement( object ); } public int indexOf( Object newElement ) { System.out.println("Set.indexOf is not allowed." ); return -1; } public Object elementAt( int index ) { return null; }

11. Inheritance In one sense, a subclass is an expansion of its superclass. a subclass can add instance variables and methods In another sense, a subclass is a contraction of its superclass. a subclass defines a subset of instances of its superclass In Java, all classes are subclasses, whether we say so or not. By default, any class that does not have an extends clause extends the class Object. Consider our Ball hierarchy: Ball, MovableBall, BoundedBall

12. public class Ball { private Rectangle location; private Color color; public Ball( int x, int y, int r ) {... } public void paint( Graphics g ) {... } public void setColor( Color newColor ) {... } protected Rectangle region() {... } protected Color color() {... } protected int radius() {... } protected int x() {... } protected int y() {... } protected void moveTo( int x, int y ) {... } } public class MovableBall extends Ball { private double dx; private double dy; public MovableBall( int x, int y, int r ) {... } public void move() {... } public void setMotion( double ndx, double ndy ) {... } protected double xMotion() {... } protected double yMotion() {... } } public class BoundedBall extends MovableBall { private Frame myWorld; public BoundedBall( int x, int y, int r, Frame myWorld ) {... } public void move() {... } }

13. Inheritance and Substitutability An object X is substitutable for an object Y if we can use X any place we use Y and the client code not know the difference. An example in practice, from the pinball games: The target vector holds any Object. That’s how Java Vectors work. We put Springs, Walls, Holes,..., into the vector. When we retrieve objects from the vector, we treat them as PinBallTargets.

14. Inheritance and Substitutability Another example in practice, from the cannon games: Our “fire” Button expects to be given an ActionListener that watches for button events. We create FireButtonListener as an implementation of the ActionListener interface. We add a FireButtonListener in place of an ActionListener. What would the alternative be?

15. Substitutability The common feature in these cases — and the key to substitutability — is that the objects share a common interface. They respond to the same messages. Inheritance and interfaces are mechanisms for ensuring the common interface.

16. Substitutability So, why write our programs so that they use substitutable objects? extendibility flexibility frameworks that implement a program’s control while allowing programmers to add new objects to the program later Of course, we can achieve these benefits without the use of inheritance and interfaces. But the compiler wouldn’t be able to help us enforce them!

17. Types of Inheritance Specialization Specification Extension Combination Limitation Construction

18. Types of Inheritance Specialization Essentially no new methods in the subclass. Most subclass methods override inherited methods. example: our BoundedBall class common in frameworks

19. 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 ) {... } }

20. Number - Abstract Class Example Parent class for numeric wrapper classes: Integer, Long, Double, etc. Subclasses must override abstract methods public abstract class Number { public abstract int intValue(); public abstract long longValue(); public abstract float floatValue(); public abstract double doubleValue() public byte byteValue() {return (byte) intValue;} public short shortValue() { return (short) intValue(); } } // end Number

21. Types of Inheritance Extension Subclass uses most or all inherited methods as-is. Subclass adds new behavior and methods. example: our MovableBall class

22. Types of Inheritance Combination 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 ) {... } }

23. 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.

24. 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.

25. 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()

26. 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.

27. 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.

28. 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 }

29. 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(); }

30. A driver class import 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() ); }

31. 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

32. Reconsidering the Design of CannonWorld Why limit our shelves to shooting cannon balls?!!!

33. 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

34. 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 (...);...} }...}

35. 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.

36. 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!

37. 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.

38. 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.

39. 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)

40. 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.

41. 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)

42. 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.

43. 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.

44. 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.

45. 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)

46. We have just converted our CannonWorld program into a framework

47. 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.

48. 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.

49. 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)

50. 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.

51. 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.

52. 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!)