Download Day 2

Design Patterns…Part 2
Elizabeth Duea
CS 5667
October 22, 2003
Recap…
• Creational patterns – concern the process
of object creation
• Structural patterns – deal with the
composition of classes or objects
• Behavioral patterns – characterize the
ways in which the classes or objects
interact and distribute responsibility
More examples
• Creational Pattern Example
- Singleton (in class Monday)
• Structural Pattern Example
- Decorator
• Behavioral Pattern Example
- Iterator
- Observer
• We played with an example of the
Singleton pattern in class on Monday.
• In a few minutes, we will some of the
homework that was turned in…
• First we will talk about a few other patterns
and then see the examples.
Decorator
• Intent: Attach additional responsibilities to
an object dynamically. Decorators provide
a flexible alternative to subclassing for
extended functionality
• AKA: Wrapper
• See handouts for more information
• Now for the example…
• Decorator adds responsibility and levels.
For example: JAVA I/O
• The InputStreamReader builds on the
functionality of being able to read basic
bytes, and provides the functionality of
being able to read characters (In Java,
characters are potentially 2 bytes).
• The BufferedReaders takes the basic
functionality of being able to read
characters and provides the ability to read
strings and also buffers the input for
efficiency.
DecoratorExample.java
import java.io.*;
public class DecoratorExample {
public static void main(String[] args) throws Throwable {
BufferedReader in =
new BufferedReader(
// Decorates the InputStreamReader
new InputStreamReader( // Decorates the InputStream
System.in));
// an InputStream
System.out.println("Please type a word or phrase");
System.out.println(in.readLine());
}
}
Iterator
• Intent: Provide a way to access the
elements of an aggregate object
sequentially without exposing its
underlying representation
• AKA: Cursor
• See handouts for more information
• Now for the example…
IteratorExample.java
import java.util.*;
public class IteratorExample {
public static void main(String[] args) {
List list = new ArrayList();
for (int i = 0; i < 10; ++i) {
list.add(new Integer(i));
}
for (Iterator i = list.iterator(); i.hasNext(); ) {
System.out.println(i.next());
}
Set set = new HashSet();
for (int i = 0; i < 10; ++i) {
set.add(new Integer(i));
}
for (Iterator i = set.iterator(); i.hasNext(); ) {
System.out.println(i.next());
}
}
}
Observer
• Intent: Define a one-to-many dependency
between objects so that when one object
changes state, all its dependents are
notified and updated automatically
• AKA: Dependents, Publish-Subscribe
• See handouts for more information
• Now for the example…
ObserverExample.java
public class ObserverExample {
public static void main(String[] args) {
Model model = new Model();
View1 view1 = new View1();
View2 view2 = new View2();
model.addObserver(view1);
model.addObserver(view2);
model.setValue(27);
model.setValue(42);
}
}
Model.java
import java.util.*;
public class Model extends Observable {
private int value;
public int getValue() {
return value;
}
public void setValue(int value) {
this.value = value;
setChanged();
notifyObservers();
}
public String toString() {
return "Example Model";
}
}
View1.java, View2.java
import java.util.*;
public class View1 implements Observer {
public void update(Observable o, Object arg) {
System.out.println("View 1 has been notified of a change in " + o);
System.out.println("New value: " + ((Model) o).getValue());
}
}
import java.util.*;
public class View2 implements Observer {
public void update(Observable o, Object arg) {
System.out.println("View 2 has been notified of a change in " + o);
System.out.println("New value: " + ((Model) o).getValue());
}
}
~
Homework
• On Monday we played with an example of
the Singleton design pattern.
• The assignment was to write a small
program using this pattern