1. Giuseppe Attardi Antonio Cisternino
Polymorphism
Giuseppe Attardi
Antonio Cisternino

2. Generic programming Code reuse is clearly a good idea
Often an algorithm can be applicable to many objects
Goal is to avoid rewriting as much as possible
Types introduce great benefits but also may limit code reuse:
int sqr(int i, int j) { return i*j; }
double sqr(double i, double j) {return i*j; }
The notion of sqr is unique but we must define it twice because of types
Languages offer mechanisms to address this problem

3. Polymorphism
The ability of associate more than one meaning to a name in a program
We have already seen two kinds of polymorphism:
Subtype/inclusion (inheritance)
Overloading
Polymorphism is the fundamental mechanism for generic programming
There are other kinds of polymorphism

4. Classification of Polymorphism
Parametric
Universal
Inclusion
Polymorphism
Overloading
Ad hoc
Coercion

5. Universal vs. ad hoc polymorphism
With overloading an implementation for each signature is required
We provide ad hoc solutions for different objects
Inheritance instead allows defining algorithms that operate on all classes of objects that inherit from a given class
In this case a single (universal) solution applies to different objects

6. Containers Example: Java Vector Signature of addElement:
Vector v = new Vector();
v.addElement("Pippo");
v.addElement(new Integer(2));
Signature of addElement:
void addElement(Object x);
The argument is of type Object because the container may contain any type of object

7. Problem with containers
Inserting an object in a vector we loose type information
In our example we implicitly upcast from String to Object:
v.addElement("Pippo");
Extracting the second element with the wrong cast produces a runtime error:
Integer i = (Integer)v.elementAt(0);

8. Weakest constraint programming
Where do we assume something about objects that we manipulate?
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 operations and arrays: operation available on all objects

9. Can we sort our vector? How to add a method for sorting a vector?
We do not have enough information on our objects: no comparison operation is available
Our vector is too generic!
Two solutions:
accept only objects that implement an interface (i.e. IComparable) that exposes a method to compare objects
public void addElement(IComparable e) {…}
Pass a functional object: an object which implements an interface for comparing Object instances (i.e. IComparator)
public void Sort(IComparator c) {…}
interface IComparator {
int compare(Object x, Object y); }

10. Abstract as much as possible!
To express generic code with subtype polymorphism we should abstract the essence of the operations required on the objects we want to manipulate
Risk is over-abstraction: once defined our vector we can’t easily add a sort method
Another issue: inheritance relies on explicit annotation of our types and changes are hard to perform

11. Iterating over a collection
A common programming pattern is to enumerate the elements of a collection
It doesn’t really matter how the collection is organized
We can implement a class per collection type whose objects enumerates the elements.
Example:
Enumeration elements() { return ???; }
void printCollection(Enumeration e) {
while (e = hasMoreElements()) {
Object o = e.nextElement();
System.out.println(o); }}
Interface

12. Question Which class implements method elements?
Class Vector
Use overloading and singleton
class Enumeration {
static Enumeration getEnumeration(Vector v){
return v.elements(); }
// Other collections’ enumerators
Thus we can add enumerators to existing collections

13. 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; // why it is not copied?

14. Is the enumerator up to date?
To ensure that the enumerator is consistent the vector should be copied into the enumerator
This isn’t reasonable: memory wasted and we iterate on a different vector!
There is no way to ensure that the enumerator is consistent with the vector
Nonetheless it is possible to do something: we introduce a “version” of the vector
Each time the vector is modified the version is incremented
The enumerator compares the version of the vector with the one of the vector when it has been created

15. Event handling in GUI
Before Java 1.1 OO GUI frameworks were based on sub-typing
GUI can be easily described using generic programming: buttons are a subtype of control which is a special window
Containers of graphical widgets operates on controls, irrespective of their types
Event dispatching and handling is dealt by virtual methods
hence by default is delegated to the super-type

16. Java AWT Event Model class Component { int x, y;
bool handleEvent(Event e); }
class Button extends Component {
String text;
bool handleEvent(Event e) { } …
class Window extends Component { … }
class Frame extends Window { … }

17. Event handling class MyButton extends Button {
boolean handleEvent(Event e) {
switch (e.type) {
case Event.MOUSE_UP: …
return true; // Event handled! }
default:
return super.handleEvent(e); }}

18. Limits of AWT Event Model
Generic programming in this case is quite elegant but inefficient
Propagation of events to a number of handlers, mostly useless
Proliferation of classes: one for each object with different behavior

19. Alternative Event Delegation model
Observer Pattern (aka publish/subscribe)
Observable has set of registered observers
Observable notifies its observers when its state changes
Handling performed by objects that provide a Listener interface (aka callback, delegate)

20. Java JDBC
Java DataBase Connectivity is a specification from Sun for accessing databases in Java
Interesting example of generic programming
It implements a driver architecture exploiting the mechanisms of JVM

21. 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:
Load the database driver (a Java class)
Create a connection to a database (using DriverManager)
Obtain a Statement and execute the query
Enumerate the rows using a ResultSet

22. JDBC example Class.forName("…"); // Load the driver
Connection c = DriverManager.getConnection("…");
Statement s = c.createStatement();
ResultSet r = s.executeQuery("select …");
while (r.hasNext()) {
// Value of second column as String
String s = r.getString(2); }

23. Question Statement is Class or Interface?
Connection, Statement are Interfaces
Through the DriverManager the program obtains an object of unknown type which implements the Connection interface
The same applies to all the other interfaces: through the connection an object implementing Statement is obtained, and so on and so forth