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POLYMORPHISM  

Generic  programming  

!  Code  reuse:  an  algorithm  can  be  applicable  to  many  objects  !  Goal  is  to  avoid  rewri:ng  as  much  as  possible  !  Example:  

int sqr(int i, int j) { return i*j; } double sqr(double i, double j) {return i*j; }

!  The  no:on  of  sqr  is  unique  but  we  must  define  it  twice  because  of  types  

!  Programming  languages  offer  mechanisms  to  address  this  problem  

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Polymorphism  

!  Polymorphism  (Greek  for  “many  forms”)  is  the  ability  for  code  to  be  used  with  values  of  different  types.    

!  For  example,  a  polymorphic  func:on  is  one  that  can  be  invoked  with  arguments  of  different  types.    

!  A  polymorphic  datatype  is  one  that  can  contain  elements  of  different  types.  

FORMS  OF  POLYMORPHISM  

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Subtype  Polymorphism  

!  Subtype  polymorphism  allows  a  term  to  have  many  types  using  the  subsump:on  rule  (inheritance).  

!  A  func:on  with  argument  τ  can  operate  on  any  value  with  a  type  that  is  a  subtype  of  τ  .  

Ad  Hoc  Polymorphism  

!  Ad-­‐hoc  polymorphism  usually  refers  to  code  that  appears  to  be  polymorphic  to  the  programmer,  but  the  actual  implementa:on  is  not.    

!  A  typical  example  is  overloading:  using  the  same  func:on  name  for  func:ons  with  different  kinds  of  parameters.    

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Ad  hoc  polymprphism  

!  There  are  actually  mul:ple  func:on  implementa:ons  (none  being  polymorphic)  and  the  compiler  invokes  the  appropriate  one.    

!  Ad-­‐hoc  polymorphism  is  a  dispatch  mechanism:  the  type  of  the  arguments  is  used  to  determine  (either  at  compile  :me  or  run  :me)  which  code  to  invoke.  

Parametric  Polymorphism  

!  Parametric  polymorphism  refers  to  code  that  is  wriPen  without  knowledge  of  the  actual  type  of  the  arguments;  the  code  is  parametric  in  the  type  of  the  parameters.    

!  Examples  include  polymorphic  func:ons  in  ML  (F#)  and  Java  (C#)  generics.  

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ML  

let rec map f l = match l with [] -> [] | x::xs -> f(x) :: map f xs;; val map : ('a -> 'b) -> 'a list -> 'b list = <fun>

map (function x -> x*10) [4;2;7];; - : int list = [40; 20; 70]

ML  

let square = (fun x -> x*x) ;; val square : int -> int = <fun>

Q: Why?

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Java    public class ColorPoint extends Point{ private final String color;

public ColorPoint(int x, int y, String color){ super(x,y); this.color = color; }

protected String makeName() { return super.makeName() + ":" + color; }

public static void main(String [] args) { System.out.println(new ColorPoint(4, 2, "viola")); } }

public class Point { protected final int x, y; private final String name;

public Point(int x, int y){ this.x = x; this.y = y; name = makeName(); }

protected String makeName() { return "["+x+", "+y+"]"; }

public final String toString() { return name; } }

Q: What is program behaviour?

Java    public class ColorPoint extends Point{ private final String color;

public ColorPoint(int x, int y, String color){ super(x,y); this.color = color; }

protected String makeName() { return super.makeName() + ":" + color; }

public static void main(String [] args) { System.out.println(new ColorPoint(4, 2, "viola")); } }

public class Point { protected final int x, y; private final String name;

public Point(int x, int y){ this.x = x; this.y = y; name = makeName(); }

protected String makeName() { return "["+x+", "+y+"]"; }

public final String toString() { return name; } }

Q: What is program behaviour? [4, 2]:null

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Java(C#)  Polymorphic  Assignment  

!  Let  S  be  an  ancestor  of  T    !  T  is  a  subclass  of  S  in  the  hierarchy  

!  Upcas:ng:  an  object  of  type  T  is  aPached  to  a  reference  of  type  S  

!  Downcas:ng:  an  object  of  type  S  is  aPached  to  a  reference  of  type  T  

!  class  Vehicle;    class  Car  extends  Vehicle;  //  car  is  a  subtype  of  Vehicle  

Vehicle  v  =(Vehicle)  new  Car();  //  upcas:ng  Car  c  =  (Car)new  Vehicle();  //  downcas:ng  

Cas:ng  in  Java  !  Upcas:ng  is  implicit    

o  For  primi:ve  types,  upcas:ng  means  assigning  a“smaller”  type  to  a  “larger”  compa:ble  type    "  byte  to  short  to  int  to  long  to  float  to  double  

(long  to  float  may  actually  lose  precision)    

o  For  reference  types,  upcas:ng  means  assigning  a  subtype  to  a  supertype,  that  is:  "  a  subclass  to  superclass    "  an  implementa:on  of  an  interface  X  to  that  interface  X    

"  an  interface  X  to  the  implementa:on  of  an  ancestor  of  X  

!  Downcas:ng  must  be  explicit    o  can  raise  run:me  excep:ons  if  it  turns  out  to  be  impossible  

o  No  casts  are  allowed  for  reference  types  outside  the  inheritance  hierarchy  

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Containers  

!  Java  Vector  Vector v = new Vector(); v.addElement("Pippo");

v.addElement(new Integer(2));

!  Vector  API:  Signature  of  addElement:  void addElement(Object x);

!  The  argument  has  type  Object  because  the  container  may  contain  any  type  of  object  

Programming  issues  

!  Inser:ng  an  object  in  a  vector  we  loose  type  informa:on  

!  In  our  example  we  implicitly  upcast  from  String  to  Object:  v.addElement("Pippo");

!  Extrac:ng  the  element  with  the  wrong  cast  produces  a  run:me  error:  Integer i = (Integer)v.elementAt(0);

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Programming  Issues  

class Vector { Object[] v; int size; public Vector() { v = new Object[15]; size = 0; } public addElement(Object e) { if (size == v.length) { Object[] w = new Object[](2 * size); w.copy(v, 0, size); v = w; } v[size++] = e; }}

!  We  assume  only  assignment  opera:ons  and  arrays:  opera:on  available  on  all  objects  

Sort  Method  

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Abstract  as  much  as  possible!  

Itera:ng  over  a  collec:on  

Interface

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Enumerator  for  Vector  class VectorEnum implements Enumeration { int idx; Vector v; bool hasMoreElements() { idx < v.size(); } Object nextElement() { return v.elementAt(idx++); } VectorEnum(Vector v) { idx = 0; this.v = v;? } }

Is  the  enumerator  up  to  date?  

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Event  handling  in  GUI  

!  Before  Java  1.1  OO  GUI  frameworks  were  based  on  sub-­‐typing  

!  GUI  can  be  easily  described  using  generic  programming:  buPons  are  a  subtype  of  control  which  is  a  special  window  

!  Containers  of  graphical  widgets  operates  on  controls,  irrespec:ve  of  their  types  

!  Event  dispatching  and  handling  is  dealt  by  virtual  methods  o  hence  by  default  is  delegated  to  the  super-­‐type  

Java  AWT  Event  Model  

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Event  handling  

Limits  of  AWT  Event  Model  

!  Generic  programming  in  this  case  is  quite  elegant  but  inefficient  

!  Propaga:on  of  events  to  a  number  of  handlers,  mostly  useless  

!  Prolifera:on  of  classes:  one  for  each    object  with  different  behavior  

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Alterna:ve  

Java  JDBC  

!  Java  DataBase  Connec:vity  is  a  specifica:on  from  Sun  for  accessing  databases  in  Java  

!  Interes:ng  example  of  generic  programming  

!  It  implements  a  driver  architecture  exploi:ng  the  mechanisms  of  JVM  

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Overall  architecture  

!  The  java.sql  package  exposes  only  interfaces  !  The  only  class  is  DriverManager  !  Using  the  class  constructor  a  driver  register  itself  

with  the  DriverManager  !  The  programmer  performs  the  following  steps:  

o  Load  the  database  driver  (a  Java  class)  o  Create  a  connec:on  to  a  database  (using  DriverManager)  o  Obtain  a  Statement  and  execute  the  query  o  Enumerate  the  rows  using  a  ResultSet  

JDBC  example