Aula 08-09 - Classes Abstratas

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Projeto de Software / P3 
Classes Abstratas 
Márcio Ribeiro 
marcio@ic.ufal.br 
twitter.com/marciomribeiro 
2 
Specific	
  capabili+es	
  
§  What	
   if	
   we	
   know	
   that	
   all	
   subclasses	
   should	
   have	
   some	
  
capability,	
   but	
   we	
   don’t	
   know	
   how	
   those	
   subclasses	
   will	
  
implement	
  it?	
  
§  There	
   would	
   be	
   no	
   code	
   that	
   could	
   be	
   wri?en	
   for	
   that	
  
capability	
  that	
  would	
  apply	
  to	
  every	
  subclass	
  	
  
3 
Different	
  taxes	
  for	
  each	
  product…	
  
§  Discounts	
  
§  TV	
  =	
  20%	
  
§  Refrigerator	
  =	
  35%	
  
§  Stove	
  =	
  15%	
  
Remember?	
  
Change	
  requests	
  all	
  the	
  +me!	
  
Abstract	
  classes	
   Product!
- brand : String"
- price : double"
+ getBrand() : String"
+ getPrice() : double"
TV!
- inches : int"
+ getInches(): int"
Refrigerator!
- size : int"
+ getSize(): int"
Stove!
- burners : int"
+ getBurners(): int"
Now the getPrice implementation is not common anymore! 
6 
What	
  about	
  new	
  products?	
  
§  How	
  to	
  force	
  they	
  will	
  have	
  getPrice?	
  
7 
Abstract	
  classes	
  
public abstract class Product {	
	
 private String brand;	
		
 protected double price;	
		
 public String getBrand() {	
 return brand;	
 }	
		
 public String toString() {	
 return "Brand = " + brand + "\nPrice = " + price;	
 }	
		
 public abstract double getPrice();	
}	
8 
Abstract	
  classes	
  
§  Methods	
   for	
   which	
   no	
   implementa+on	
   in	
   the	
   superclass	
  
makes	
  sense	
  should	
  be	
  declared	
  abstract	
  	
  
§  Abstract	
   means	
   the	
   method	
   has	
   no	
   defini+on,	
   only	
  
declara+on	
  in	
  the	
  superclass	
  
§  So,	
  we	
  cannot	
  instan+ate	
  an	
  abstract	
  class!	
  
	
  
§  To	
  ensure	
  that	
  a	
  subclass	
  does,	
  indeed,	
  override	
  a	
  method	
  
Abstract	
  classes	
   Product!
- brand : String"
- price : double"
+ getBrand() : String"
+ getPrice() : double!
TV!
- inches : int"
+ getInches(): int"
+ getPrice() : double"
Refrigerator!
- size : int"
+ getSize(): int"
+ getPrice() : double"
Stove!
- burners : int"
+ getBurners(): int"
+ getPrice() : double"
Exercises	
  
11 
Shape	
  
§  Suppose	
  we	
  want	
   to	
  design	
  a	
   system	
   that	
  draws	
   shapes	
   in	
  
the	
   screen.	
   Consider	
   three	
   shapes:	
   triangle,	
   rectangle,	
   and	
  
circle.	
  All	
  shapes	
  must	
  have	
  capabili+es	
  to	
  print	
  themselves	
  
and	
  to	
  calculate	
  their	
  areas.	
   In	
  addi+on,	
  each	
  shape	
  should	
  
have	
   an	
   specific	
   point.	
   This	
   point	
   is	
   important	
   to	
   start	
  
drawing	
  the	
  shape	
  in	
  the	
  screen.