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Question of the Week Archive

Topic:   How do I signal another object?

Question:  Can anyone point me in the right direction when it comes to event-handling and signaling. I have two objects (non-AWT) and one needs to 'register an interest' in a particular event that the other one is signaling. The object that is 'listening' for the event from the second object has a handle to it, but I'm a little unsure how the second one 'signals' and what the first one should do in order to handle the event. Can anyone help or point to some useful documentation? Thanks Dave

Answer:

There are a few models to explore. The Observer/Observable pattern may be appropriate in the situation where you have need to update a set of objects based on a change in another. At least two objects are needed, one which extends the Observable class and represents the changeable 'data' or 'model' and one that implements the Observer interface and is the class whose interest is registered with the Observable. The following is a short demo - the only code of real import is the implementation of the update() method by the Observer, and the use of the three methods addObserver(),setChanged(), and notifyObservers() [the minimum that must be used] by the Observable. A change in the state of a String in the Observable will notify the Observer, which will in turn output a message to that effect to the console:

import java.util.*; 
import java.io.*; 

public class Actor extends Observable { 

    private String stateString = "idle"; 

    public Actor() { 
	addObserver(new Watcher()); 
    } 

    public String getState() { 
	return stateString; 
    } 

    public void setState(String s)  { 
	if( ! (s.equals(stateString) ) ) { 
	    setChanged(); 
	    notifyObservers("State changed"); 
	} 
	stateString = s; 
    } 
    
    public static void main(String[] args) { 
	String s; 
	Actor actor = new Actor(); 
	BufferedReader reader = 
	    new BufferedReader(
	     new InputStreamReader(System.in)); 
	try { 
	    System.out.println("Enter a state (" + 
		actor.getState(
		 ) + " is the current state): "); 
	    while((s = reader.readLine()) != null) { 
		actor.setState(s); 
		System.out.println("Current state: " +
		  actor.getState()); 
	    } 
	} 
	catch(Exception e) { 
	    System.out.println("Program terminated."); 
	} 
    } 
} 


class Watcher implements Observer { 
  public void update(Observable o, Object arg) { 
   System.out.println("Observable object " +
    o + " has changed state."); 
    } 
} 

Many thanks to JDC member xhunterx for contributing to this answer.

[Member not logged in]
Is there any way to cast a Thread returned by an enumerate from the class ThreadGroup in order to make full use of polymorphism. I get a runtime error if I do that.

Sat Oct 16 17:29:54 PDT 1999

MChaston
One thing to watch out for in this case is the update sequence. If the observer reads the 'stateString' on notification, because it is updated after the notification message, it would be wrong. It is normally safer to update the system before firing change notifications. You may also want to look at the java.awt.EventListener interface and associated models (see Java Tutorials).

Sun Oct 17 06:23:41 PDT 1999
tenne hjkjhkjhkj

Sun Oct 17 17:14:29 PDT 1999 [Member not logged in] question for the for loop program; how do I promt for a number of value;calculate the maximun input and the average input the interger number is 5;(loop 5times); for loop=20times what is the right code for the program loop?

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