Lecture 5

Lecture Topic:

• Class(Point, Ray)
• Null
• Operators
• Casting
• Extending a Class
  
• Quiz

What's class point?

-- it has two data members that keeps its
location, and some functions that tell its attributes(location, magnitude...).
class Point { 
  

public double x, y; final double PI = Math.PI; Point() {x=0; y=0}

Point(int x1; int y): x(x1),y(y1) {} double magnitude() { return Math.sqrt(x*x

+ y*y); } void print() { system.out.print(this.getclass().getstring() +

"="); system.out.println(x + " " + y); } }

What's class ray?

-- It is a line between two points, so it
has two points and function which tells if it intersects with another ray.
class Ray { Point p1, p2; point intersect(Ray r); } Point Ray::intersect(Ray
r) { 
  

Point p3 = r.p1; Point p4 = r.p2; double a1, b1, c1, a2, b2, c2; a1

= p2.y - p1.y; b1 = p1.x - p2.x; .... Point pi = new Point(..., ... ); return

pi; }

NULL

-- null value can be used to detect whether
an object has an viable instance.
if (user.object != NULL)
system.out.println("Object exist"); 

Operators

"==" means equal / instance of
Syntax: 
  

if (User.Object instance of <Class>) { ... }

Example: 
  

if (P1 instance of Point) { system.out.println(this.getclass(P1).getstring

+ "is a point"); }

Casting

-- is data type conversion. In class casting,
you should only winden a class by casting it to a superclass. public
class simpleDraw extends Applet { Vecter drawnShapes; void paint(Graphics
g) { 
  

raw_grid(g); hape s; nt numshapes = drawnShapes.size(); or (int i=0;

i<numshapes; i++) s = (Shape)drawnShapes.elementAt(i); s.draw(g); }

}
 

Extending a class

1>. If this class is a subclass of another
class, use extends to indicate the superclass of this class: Syntax:
class myClassName extends mySuperClassName { ... } Example:
class Target extends Shape { 

void draw(graphics g ) { g.setcolor(this.color);
fill_circle(g, Pi, Shape.radius); } 

} 2>.
If this class implements a specific interface, use implements to refer to
that interface: Syntax: class MyRunnableClassName implements Runnable
{ ... }
 

Sample Quiz

 
1. What does class extension do?
 
  a. adding more classes.
 
  b. define more features to a existing class.
 
  c. new class in java can be extensions of any single existing class.