April 27, 1998 CS102-02 Lecture 5-1

Object-Oriented Programming Revisited

CS 102-02

Lecture 5-1

The Wonderful World of Objects

April 27, 1998 CS102-02 Lecture 5-1

A Brief Outline of Today’s Events

• Review of inheritance• protected members

• Subclasses and superclasses

April 27, 1998 CS102-02 Lecture 5-1

Inheritance

• Doesn't mean that classes are born with a silver spoon in their mouths

• The is-a relationship

• The world's objects fall into broad categories– Categories depend on your perspective

April 27, 1998 CS102-02 Lecture 5-1

Broad Means General

Level of detail

Number of matching objects

java.lang.Object

java.awt.Component

java.awt.Container

java.awt.Panel

java.applet.Applet

April 27, 1998 CS102-02 Lecture 5-1

A Little Terminology• Component is a direct

subclass of Object• Container is the

direct superclass of Panel

• Container is a superclass of Applet

java.lang.Object

java.awt.Component

java.awt.Container

java.awt.Panel

java.applet.Applet

April 27, 1998 CS102-02 Lecture 5-1

The Role of Inheritance

• Found an is-a relationship in the world? Use inheritance because:– Models the world well– Code is more extensible and flexible (For

example, create another Container without worrying about Panel stuff)

• Reuse!

April 27, 1998 CS102-02 Lecture 5-1

Inheriting in Java

• Use extends to indicate the immediate superclass

• A class can only extend one other class (single inheritance)– C++ allows multiple inheritance– Java allows multiple interface inheritance

• Without extends, a class extends java.lang.Object

April 27, 1998 CS102-02 Lecture 5-1

Hiding Information

• Information hiding (a.k.a. encapsulation) is fundamental to OOP– Can't see information you don't need

• Access modifierspublic: everyone can see everything

private: only the class itself can see anything

and the new kid on the block...

April 27, 1998 CS102-02 Lecture 5-1

protecting Your Information

• If you mark an item protected, the item is available to:– The class itself– Other classes in the same package– Subclasses in other packages

April 27, 1998 CS102-02 Lecture 5-1

Accessor RoundupMember Visibility

Accessibleto:

public protected package private

Sameclass Yes Yes Yes Yes

Class insamepackage

Yes Yes Yes No

Subclassin anotherpackage

Yes Yes No No

Not asubclass,anotherpackage

Yes No No No

April 27, 1998 CS102-02 Lecture 5-1

Class Identity Crisis

• Subclass objects are more specific versions of superclass objects– Can always treat a subclass object as a

superclass object– Employee example

A Senior Vice-President object and a Customer Service Representative object are both Employee objects

April 27, 1998 CS102-02 Lecture 5-1

When is Superclass Object Also a Subclass Object?

public class PointData members: protected int x, y;

public class Circle

Data members:

protected double radius;

April 27, 1998 CS102-02 Lecture 5-1

A Class Quiz

• Can we do this?

Point ryersonHall = new Point(12, 15);

Circle buckingham = new Circle();

Circle roundAndRound[] = new Circle[10];

roundAndRound[0] = buckingham;

roundAndRound[1] = ryersonHall;

April 27, 1998 CS102-02 Lecture 5-1

The Test Applet

public class Test extends Applet {private Point pointRef, p; private Circle circleRef, c;

public void init() { p = new Point( 30, 50 );

c = new Circle( 2.7, 120, 89 ); }:

• Points are just (x, y)

• Circles are points with a radius (r)

April 27, 1998 CS102-02 Lecture 5-1

Circles & Points

// Attempt to treat a Circle as a PointpointRef = c; // assign Circle to pointRefg.drawString( "Circle c (via pointRef): " + pointRef.toString(), 25, 70 );

• Assign a subclass reference to a superclass reference– A Circle object is a Point object

• Can't call Circle-specific methods or data with pointRefpointRef.setRadius(9.37); // Compile-time error

Error: Test.java(26): Method setRadius(double) not found in Point

April 27, 1998 CS102-02 Lecture 5-1

Circles & Points

// Treat a Circle as a Circle (with some casting)circleRef = (Circle) pointRef; // cast super to subg.drawString( "Circle c (via circleRef): " +

circleRef.toString(), 25, 100);g.drawString( "Area of c (via circleRef): " +

precision2.format( circleRef.area() ), 25, 115 );

• Convert a superclass reference to a subclass reference– Must use an explicit cast

• Remember that pointRef references a Circle at this point

April 27, 1998 CS102-02 Lecture 5-1

Circles & Points

// Attempt to refer to Point object// with Circle referencecircleRef = (Circle) p; // line 39 in Test.java

• Remember that p is still a Point object

• Java knows that p references a Point, and not a Circle– Throws a CastClassException

April 27, 1998 CS102-02 Lecture 5-1

Building Super Subclasses

• Use super to refer to the parent class

• Circle is-a Point plus a radius– Let the Point class build the Point, and

then Circle can add the radiuspublic Circle( double r, int a, int b ) {

super( a, b ); // Ask Point class to build

// a point

setRadius( r ); // Set the radius ourselves

}

April 27, 1998 CS102-02 Lecture 5-1

Being a super user• Don't have to use super

– If you don’t, the superclass no-argument constructor is called automatically

– If superclass' no-arg constructor isn't defined, it's an error

public Circle() {

// Calls no-arg constructor of Point

setRadius(0.0)

}

April 27, 1998 CS102-02 Lecture 5-1

Using super II

• If you do use super, it's gotta be in the first line of the constructor

public Circle( double r, int a, int b ) {super( a, b ); // Ask Point class to build

// a pointsetRadius( r ); // Set the radius ourselves

}