Polymorphism

Giuseppe Attardi

Antonio Cisternino

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

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

Classification of Polymorphism

Polymorphism

Universal

Ad hoc

Parametric

Inclusion

Overloading

Coercion

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

Containers

Example: Java VectorVector 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

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);

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

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 objectspublic 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); }

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

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

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

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? }}

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

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

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 { … }

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);

}}

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

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)

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

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

JDBC example

Class.forName("…"); // Load the driverConnection 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);}

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