// CS U213 Spring 2007
// Lecture 14: February 8, 2007

/*

Goals:

 - Extending classes and overriding methods

Introduction:

Sometimes it is uselful to represent a special case of our data
as a subclass of a more general class. This way we can use a uniform 
way to manipulate our data and in the same time we are able to treat
in a unique way any special case. This can be done by overriding 
the superclass' s methods inside the new subclass.


Let's consider the following example:


                     +-----------------------------+
                     |             Star            | 
                     +-----------------------------+
               +---->|Posn loc                     |
               |     |int  lifespan                |
               |     +-----------------------------+
               |     |Star move()                  |
               |     |boolean draw(Canvas C)       |
               |     |boolean iswithin(int w,int h)|   
               |     +-----------------------------+
               |
     +------+  |
     | Posn |--+
     +------+   
     |int x |
     |int y | 
     +------+



We want a burned star, e.g. a star with 0 to move faster than a live star.
So we implement move as follows:


//move a star
Star move(){
 if lifespan-1>0
  return (new Star(new Posn(this.loc.x,this.loc.y+10),this lifespan-1)
 else  
  return (new BurnedStar(new Posn(this.loc.x,this.loc.y+10)));}

We want a burned star to have differnt collor than a live star.
So we implement draw as follows:

//draw a star
boolean draw(Canvas c){
 if lifespan>0
  return (c.drawDisk(this.loc,3,new Yellow())
 else  
  return (c.drawDisk(this.loc,3,new Yellow()))}


------*Alternative approach*-----------------

--Use a subclass

We can use a subclass of stars to represent a burned Star
and thus avoid all the if checking. Let Java worry about that.


Let's how we can do that.

Check the following class diagram:

                     +-----------------------------+
                     |             Star            | 
                     +-----------------------------+
               +---->|Posn loc                     |
               |     |int  lifespan                |
               |     +-----------------------------+
               |     |Star move()                  |
               |     |boolean draw(Canvas C)       |   
               |     +-----------------------------+
               |                /_\
     +------+  |                 |
     | Posn |--+           +-----------+
     +------+              |BurnedStar | 
     |int x |              +-----------+
     |int y |              |           |
     +------+              +-----------+
                           |           |
                           +-----------+



Now we can implement again the two previous functions:

//In Star
//move a star
Star move(){
  if (lifetime-1>0)
    return new Star(new Posn(this.loc.x,this.loc.y+10),this.lifespan-1);
   else 
    return new BurnedStar(new Posn(this.loc.x,this.loc.y+10));}


//In BurnedStar
//move a burned tar
Star move(){
 return new BurnedStar(new Posn(this.loc.x,this.loc.y+20));}


//In Star
//draw a star
boolean draw(Canvas c){
  return c.drawDisk(this.loc,3,new Black());}

//In BurnedStar
//draw a burned star
boolean draw(Canvas c){
  return c.drawDisk(this.loc,3,new Yellow());}

*/
//draw teachpack

import draw.*;
import colors.*;
import geometry.*;

// to represent a Star
class Star{

 Posn loc;
 int  lifetime;

 Star(Posn loc,int lifetime){
  this.loc=loc;
  this.lifetime=lifetime;}

//move a star
Star move(){

   if (lifetime-1>0)
    return new Star(new Posn(this.loc.x,this.loc.y+10),this.lifetime-1);
   else 
    return new BurnedStar(new Posn(this.loc.x,this.loc.y+10));}


//draw a star
boolean draw(Canvas c){
  return (c.drawDisk(this.loc,3,new Yellow()));}


 }

//to represent a BurnedStar
class BurnedStar extends Star{


  BurnedStar(Posn loc){
   super(loc,0);}


//move a burned star
Star move(){
 return new BurnedStar(new Posn(this.loc.x,this.loc.y+20));}

//draw a burned star
boolean draw(Canvas c){
  return  (c.drawDisk(this.loc,3,new Black()));}

 }


class Examples{

 Canvas board = new Canvas(100,100);

 Posn a = new Posn(50,15);
 Star s0=new Star(a,2);
 Star s1=this.s0.move();
 Star s2=this.s1.move();
 Star s3=this.s2.move();
 boolean init = this.board.show();

 boolean show0 = check s0.draw(board) expect true;
 boolean show1 = check s1.draw(board) expect true;
 boolean show2 = check s2.draw(board) expect true;
 boolean show3 = check s3.draw(board) expect true;
}