Chapter 18: The Observer Pattern
Code Examples

Example 18-1: Observer Implemented

TOC

The Observer pattern aids flexibility and keeps things decoupled

Example 18-1: Observer Implemented

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// Note: I don't use the Java Observer or Observable.
// In practice they give me little assistance and I
// don't like their interfaces.
import java.util.*;
public class Customer {
    private static Vector myObs;
    static {
        myObs= new Vector();
    }
    public static void attach(MyObserver o){
        myObs.addElement(o);
    }
    public static void detach(MyObserver o){
        myObs.remove(o);
    }
    public String getState () {
        // have other methods that will give the 
        // required information
        // Here, just return null so this will compile 
        return null;
    }
    public void notifyObs () {
        // set arg to something that helps
        // tell the Observers what happened
        for (Enumeration e = myObs.elements(); e.hasMoreElements() ;) {
            ((MyObserver) e).update(this);
        }
    }
}
interface MyObserver {
    void update (Customer myCust);
}
class AddrVerification implements MyObserver {
    public AddrVerification () {
    }
    public void update (Customer myCust) {
        // do Address verification stuff here
        // can get more information about customer
        // in question by using myCust
    }
}
class WelcomeLetter implements MyObserver {
    public WelcomeLetter () {
    }
    public void update (Customer myCust) {
        // do Welcome Letter stuff 
        // here can get more 
        // information about customer 
        // in question by using myCust
    }
}