Lecture 6

Lecture Topic:

Casting, Subclassing, Abstract classing and No No's in Java

Any object goes into an instance of a vector

Example

Shape S;

S = (Shape) DrawnShape.elementAL( i );

Extending a class

A superclass is extended by a subclass

Shape(superclass)

Camera(subclass)
Target(subclass)
Laser(subclass)
Print(subclass)

Polymorphism -> Greek " many shapes "

send the same message to any subclass

Example

class Print{: static.void.ln( ){
OS.Println( );
}
static .void.ln( double d ){: OS.Println( d );
}

Name Collision

Variable and method names in a subclass override those in a super class

if variable may not be equal names, then there is a syntax error: name collision

Inheritance n/s

Nesting

a student has a grade

a student is a person

class student extends person{: Grade grade = new Grade( );

Abstraction virtual methods C++ make a class abstract to force subclassing

Abstract class can not be instanced!

abstract class Shape{: void Print( ){
system.out.println( "class" + this.getClass( ).getName( ) );
}
and methods you must overwrite
abstract void draw( Graphics g );

you may not make an instance of an abstract class
you may not make an instance of a subclass before implementing abstract methods

class < class name >: [extends < class_name > ]
[implement < interface_name > ]
{: [<variable declaration>]
[<method declaration>]
}
final class Camera extends Shape{ }: class Pentax extends Camera{ }// illegal, because class Camera is final
It is final!

you may not extend a final class
you may not override a final variable of method
you may not set the length field method of a array

Pentium bug

find the bug( a C++ 2.0 patch from Microsoft should fix it!)

Example

public class Pentium{: public static void main( string args[ ] ){
double x = 41195835.0;
double y = 3145727.0;
double z = x - (x/y) * y;
system.out.println(z);
}
}
Java pentium is busted : 256
The result should be 0.000

Applets

If a java program has main in it,

it is an application.

Else,

it is an applet.

Applets are emerged in Web pages

application run stand-alone

public class SimpleDraw extends Applet{//SimpleDraw is a class name here: global variable;
public static void main(string args[ ]){
SimpleDraw Sd = new SimpleDraw( );//SimpleDraw is an applet name here
Sd.init( );
}
}

Sample Quiz

1.If a Java program has main in it, it is an application.

True

False

2.An abstract class can not be instanced

True

False