Intent:
"Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically."
Use case:
While any customer enter into system :
* send a welcome email
* verify the customer's address.
UML Diagram:
Code:
package com.jim.observer;
import java.util.Enumeration;
import java.util.Vector;
class Customer {
static private Vector<Observer> myObs;
static {
myObs = new Vector<Observer>(); //ObserverCellection
}
private String id;
public String getId() {
return id;
}
public Customer(String id) {
this.id = id;
}
public static void reg(Observer o) {
myObs.addElement(o);
}
public static void unreg(Observer o) {
myObs.remove(o);
}
//notify all observers
public void notifyObs() {
for (Enumeration<Observer> e = myObs.elements(); e.hasMoreElements();) {
((Observer) e.nextElement()).update(this);
}
}
}
abstract class Observer {
public Observer() {
Customer.reg(this); //register observer
}
abstract public void update(Customer myCust);
}
class WelcomeLetter extends Observer {
public WelcomeLetter() {
super();
}
public void update(Customer myCust) {
System.out.println("has sent welcome email to " + myCust.getId());
}
}
class AddrVerify extends Observer {
public AddrVerify() {
super();
}
public void update(Customer myCust) {
System.out.println("has verifed address of " + myCust.getId());
}
}
public class ObTest {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Customer cust1 = new Customer("cust1");
Customer cust2 = new Customer("cust2");
Observer wel = new WelcomeLetter();
Observer add = new AddrVerify();
cust1.notifyObs();
cust2.notifyObs();
}
}Output:
has sent welcome email to cust1
has verifed address of cust1
has sent welcome email to cust2
has verifed address of cust2
has verifed address of cust1
has sent welcome email to cust2
has verifed address of cust2