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| Line 6: |
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| * Modules are difficult to understand in isolation. | | * Modules are difficult to understand in isolation. |
| * Modules are difficult to reuse or test because dependent modules must be included. | | * Modules are difficult to reuse or test because dependent modules must be included. |
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| = The Observer pattern =
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| as an example in Lecture 23. But we've really only scratched the surface on what there is to know about achieving low coupling.
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|
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| Browse the Web and the ACM DL for other information, both
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|
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| = Observer Pattern Example =
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|
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| Here is an example that takes keyboard input and treats each input line as an event. The example is built upon the library classes <tt>java.util.Observer</tt> and <tt>java.util.Observable</tt>. When a string is supplied from System.in, the method <tt>notifyObserver</tt> is then called, in order to notify all observers of the event's occurrence, in the form of an invocation of their 'update' methods - in our example, <tt>ResponseHandler.update(...)</tt>.
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|
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| The file myapp.java contains a main() method that might be used in order to run the code.
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|
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| /* File Name : EventSource.java */
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|
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| package OBS;
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| import java.util.Observable; //Observable is here
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| import java.io.BufferedReader;
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| import java.io.IOException;
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| import java.io.InputStreamReader;
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|
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| public class EventSource extends Observable implements Runnable
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| {
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| public void run()
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| {
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| try
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| {
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| final InputStreamReader isr = new InputStreamReader( System.in );
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| final BufferedReader br = new BufferedReader( isr );
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| while( true )
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| {
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| final String response = br.readLine();
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| setChanged();
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| notifyObservers( response );
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| }
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| }
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| catch (IOException e)
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| {
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| e.printStackTrace();
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| }
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| }
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| }
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| </source>
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|
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| /* File Name: ResponseHandler.java */
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|
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| package OBS;
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|
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| import java.util.Observable;
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| import java.util.Observer; /* this is Event Handler */
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|
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| public class ResponseHandler implements Observer
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| {
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| private String resp;
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| public void update (Observable obj, Object arg)
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| {
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| if (arg instanceof String)
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| {
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| resp = (String) arg;
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| System.out.println("\nReceived Response: "+ resp );
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| }
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| }
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| }
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| </source>
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|
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| /* Filename : myapp.java */
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| /* This is main program */
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|
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| package OBS;
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|
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| public class myapp
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| {
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| public static void main(String args[])
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| {
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| System.out.println("Enter Text >");
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|
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| // create an event source - reads from stdin
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| final EventSource evSrc = new EventSource();
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|
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| // create an observer
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| final ResponseHandler respHandler = new ResponseHandler();
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|
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| // subscribe the observer to the event source
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| evSrc.addObserver( respHandler );
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|
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| // starts the event thread
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| Thread thread = new Thread(evSrc);
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| thread.start();
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| }
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| }
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| </source>
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|
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| Also refer to an article in JavaWorld
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| http://www.javaworld.com/javaworld/javaqa/2001-05/04-qa-0525-observer.html
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| [[CSC/ECE 517 Summer 2008/wiki3 Assignment|Back to the assignment page]] | | [[CSC/ECE 517 Summer 2008/wiki3 Assignment|Back to the assignment page]] |