ashleycoker
asked on
Implementing the Observable Interface
I am trying to write a simple application which works like this: It has a server program and a client program. The server must accept multiple connections from clients over a network. Once connected, the clients have a simple window in which every line they type is repeated on all other client windows. The server maintains the state of the current line, and when it is changed by one of the clients all other clients are notified and update their screen. This program is simply for research and so I can learn the Java language. I need to use the Observable() interface. This is what I have wrtiiten so far, I am new to programming and dont know how to create observers:
import java.io. *;
import java.net. *;
import java.util.*;
public class MessageServer extends Observable{
private String Latest = ""; //This is the latest line entered from one of the clients
/** Creates a new instance of MessageServer */
public MessageServer() {
}
/**
* @param args the command line arguments
*/
public static void main(String[] args) throws IOException{
ServerSocket myServerSocket = null;
try{
myServerSocket = new ServerSocket(1234);
} catch (IOException e) {
System.err.println("Error listening on port 1234");
System.exit(-1);
}
while(true)
{
//Accept the connection request
new MessageClientThread(myServ
//Create a thread to deal with the Client
}
}
}
As you can see the program accepts connections and starts threads of MessageClientThread. These were going to be observers of this program which should extend the Observable interface. Every time the String Latest at the top of the program changes they should update. However, I dont know how to create Observers. i have tried changing the line:
new MessageClientThread(myServ
to:
addObserver(new MessageClientThread(myServ
but I get the error:
non-static method addObserver(java.util.Obse
I suppose me not understanding this comes from my lack of knowledge of OO programming. How if I have a main routine, (where the program starts) can I create objects just using the new keyword like this:
new MessageClientThread(myServ
Surely if i do this then I have no handle (is this the right word) on the object, i.e. how do I reference it in the future? I thought you had to do something liike this:
MessageClientThread MyObject = new MessageClientThread(myServ
Thanks guys, much appreciated
Ashley
import java.io. *;
import java.net. *;
import java.util.*;
public class MessageServer extends Observable{
private String Latest = ""; //This is the latest line entered from one of the clients
/** Creates a new instance of MessageServer */
public MessageServer() {
}
/**
* @param args the command line arguments
*/
public static void main(String[] args) throws IOException{
ServerSocket myServerSocket = null;
try{
myServerSocket = new ServerSocket(1234);
} catch (IOException e) {
System.err.println("Error listening on port 1234");
System.exit(-1);
}
while(true)
{
//Accept the connection request
new MessageClientThread(myServ
//Create a thread to deal with the Client
}
}
}
As you can see the program accepts connections and starts threads of MessageClientThread. These were going to be observers of this program which should extend the Observable interface. Every time the String Latest at the top of the program changes they should update. However, I dont know how to create Observers. i have tried changing the line:
new MessageClientThread(myServ
to:
addObserver(new MessageClientThread(myServ
but I get the error:
non-static method addObserver(java.util.Obse
I suppose me not understanding this comes from my lack of knowledge of OO programming. How if I have a main routine, (where the program starts) can I create objects just using the new keyword like this:
new MessageClientThread(myServ
Surely if i do this then I have no handle (is this the right word) on the object, i.e. how do I reference it in the future? I thought you had to do something liike this:
MessageClientThread MyObject = new MessageClientThread(myServ
Thanks guys, much appreciated
Ashley
myobservable.addObserver(m
You use the above to register Observer objects that are intested in beng notified when the Observable object changes.
Then when your Observable object makes a change you call notifyObservers() and this will call the update() method of all Observer objects that are listening.
Then when your Observable object makes a change you call notifyObservers() and this will call the update() method of all Observer objects that are listening.
eg. if you want your client threads to be observers of MessageServer you would do something like:
MessageClientThread client = new MessageClientThread(myServ
addObserver(client);
client.start();
MessageClientThread client = new MessageClientThread(myServ
addObserver(client);
client.start();
ASKER
This is my code as you said:
public class MessageServer extends Observable {
private String Latest = "";
/** Creates a new instance of MessageServer */
public MessageServer() {
}
/**
* @param args the command line arguments
*/
public static void main(String[] args) throws IOException{
ServerSocket myServerSocket = null;
try{
myServerSocket = new ServerSocket(1234);
} catch (IOException e) {
System.err.println("Error listening on port 1234");
System.exit(-1);
}
while(true)
{
//Accept the connection request
//new MessageClientThread(myServ
//Create a thread to deal with the Client
MessageClientThread client = new MessageClientThread(myServ
addObserver(client);
client.start();
}
}
}
but i get the error:
MessageServer.java [44:1] non-static method addObserver(java.util.Obse
public class MessageServer extends Observable {
private String Latest = "";
/** Creates a new instance of MessageServer */
public MessageServer() {
}
/**
* @param args the command line arguments
*/
public static void main(String[] args) throws IOException{
ServerSocket myServerSocket = null;
try{
myServerSocket = new ServerSocket(1234);
} catch (IOException e) {
System.err.println("Error listening on port 1234");
System.exit(-1);
}
while(true)
{
//Accept the connection request
//new MessageClientThread(myServ
//Create a thread to deal with the Client
MessageClientThread client = new MessageClientThread(myServ
addObserver(client);
client.start();
}
}
}
but i get the error:
MessageServer.java [44:1] non-static method addObserver(java.util.Obse
Move all the code in main into your ctor.
And in replace your main with:
public static void main(String[] args) throws IOException{
new MessageServer();
}
And in replace your main with:
public static void main(String[] args) throws IOException{
new MessageServer();
}
ASKER
Thanks a bunch, at least it compiles now, but why was it giving that error?
main is a static method, which means it does not have an object instance associated with it. addObbserver() needs to be called on a object instance.
ASKER
Now I have done that, how do I get the clients to send a message to the server to update it, and the get the server to notifyObservers() . How can the server notifyObservers when it is updated if it is in a loop waiting for client connections?
Cheers
Ashley :-)
Cheers
Ashley :-)
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ASKER
I realise to implement Observers you create a class that implements the 'Observer' interface, but how do instances of this class know what object to Observe? Where in what class do you say, 'I want this object to observe me'!
I am confused as usual!!
Thanks
Ashley :-)