Session 18

Chapter 8: Understanding Inheritance

Recall Exercise 2 From Tuesday

• It’s very annoying to move a target from the pallet and drop it in the wrong place.

• So, we extended the solution to allow for the targets in the playing field to be moved.

Exercise 2 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 Ball( e.getX(), e.getY(), 1 );

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

Opening Exercise 2 Solution• We didn’t want to create a PinBall because PinBalls

get constructed with motion and a large size.• This forced us to change the PinBallTarget interface to

accept the more general Ball: public interface PinBallTarget {

public boolean intersects( Ball aBall );

public void moveTo ( int x, int y );

public void paint ( Graphics g );

public void hitBy ( PinBall aBall );

} // end PinBallTarget

• We also have to change the intersects methods in each of the targets

• It turns out that we could have used a PinBall since it exists for such a short time

Question

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

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”

InheritanceWhat?

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.

Another ExerciseOn 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?

Another ExerciseOn 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

Another ExerciseOn 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.

Another ExerciseOn 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?

A possible solutionimport 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;}

}

InheritanceIn 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

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() { ... }

}

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.

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?

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.

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!

Types of Inheritance

Specialization• Essentially no new methods in the subclass.• Most subclass methods override inherited

methods.

• 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• example: pinball targets

private class MouseKeeper extends MouseAdapter {private PinBallTarget element;public void mousePressed ( MouseEvent e ) { ... }public void mouseReleased( MouseEvent e ) { ... }

}

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

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.