Observer
Observer Pattern is pretty much like newspaper subscriptions. The Publisher notifies subscribers automatically when any state changes.
import java.util.ArrayList;
class App {
public static void main(String[] args) {
Publisher feed = new Publisher();
Bank ING = new Bank();
Bank BNP = new Bank();
feed.subscribe(ING);
feed.subscribe(BNP);
feed.setRates();
feed.notifyObservers(); // Look Here
}
}
interface Observer {
public void update(String rate);
}
interface Subject {
public void subscribe(Observer o);
public void unsubscribe(Observer o);
public void notifyObservers();
}
class Publisher implements Subject {
ArrayList<Observer> observers = new ArrayList<>();
String rate;
public void setRates() { rate = "EURUSD:1.2222"; }
public void subscribe(Observer o) { observers.add(o); }
public void unsubscribe(Observer o) { observers.remove(o); }
public void notifyObservers() {
for(Observer o : observers) { // Look Here
o.update(rate);
}
}
}
class Bank implements Observer {
public void update(String rate) { // Look Here
System.out.println(this + ": " + rate);
}
}
/**
Bank@4d7e1886: EURUSD:1.2222
Bank@3cd1a2f1: EURUSD:1.2222
**/
Observers can subscribe/unsubscribe themselves v3.4
import java.util.ArrayList;
/**
* Observer subscribe/unsubscribe themselves
* Subject Instance variable (feed) is passed as parameter
*/
class App {
public static void main(String[] args) {
Publisher feed = new Publisher();
Bank ING = new Bank("ING", feed);
Bank BNP = new Bank("BNP", feed);
feed.updateRates("Monday");
BNP.unsubscribe();
feed.updateRates("Tuesday");
}
}
interface Observer {
public void update(String rate);
public String getName();
}
interface Subject {
public void subscribe(Observer o);
public void unsubscribe(Observer o);
public void notifyObservers();
}
class Publisher implements Subject {
ArrayList<Observer> observers = new ArrayList<>();
String rate;
public void updateRates(String day) {
if (day == "Monday") { rate = "EURUSD:1.2222"; }
if (day == "Tuesday") { rate = "EURUSD:1.4444"; }
System.out.println("CentralBank: [Rates updated] *");
notifyObservers();
}
public void subscribe(Observer o) {
observers.add(o);
}
public void unsubscribe(Observer o) {
observers.remove(o);
}
public void notifyObservers() {
for(Observer o : observers) { // Look Here
o.update(rate);
System.out.println("CentralBank: [" + o.getName() + " notified]");
}
}
}
class Bank implements Observer {
private String name;
private Subject feed;
private String rate;
public Bank(String n, Subject f) {
name = n;
feed = f;
feed.subscribe(this); // client subscribe
}
@Override public String getName() {
return name;
}
@Override public void update(String r) { // Look Here
rate = r;
System.out.print(" " + name + ": " + rate + " / ");
}
public void unsubscribe() {
feed.unsubscribe(this);
System.out.println(name + ": I've [unsubscribed] from Rates Feed!");
}
}
/*
CentralBank: [Rates updated] *
ING: EURUSD:1.2222 EURGBP:0.3333 / CentralBank: [ING notified]
BNP: EURUSD:1.2222 EURGBP:0.3333 / CentralBank: [BNP notified]
BNP: I've [unsubscribed] from Rates Feed!
CentralBank: [Rates updated] *
ING: EURUSD:1.4444 EURGBP:0.6666 / CentralBank: [ING notified]
*/
A listener is an implementation of observer pattern.
import java.util.ArrayList;
class App {
public static void main(String[] args) {
Service service = new Service();
service.addListener(new MessageListener() { // observer
@Override public void onMessageReceived(String message) {
System.out.println("Client received message: " + message);
}
@Override public void onDisconnect() {
System.out.println("Client disconnected");
}
});
service.sendMessage("order_id 120");
service.notifyListeners();
service.sendMessage("disconnect");
service.notifyListeners();
}
}
interface MessageListener {
public void onMessageReceived(String message);
public void onDisconnect();
}
interface Subject {
public void addListener(MessageListener o);
public void notifyListeners();
}
class Service implements Subject {
ArrayList<MessageListener> observers = new ArrayList<>();
private String message;
public void addListener(MessageListener o) { observers.add(o); }
public void removeListener(MessageListener o) { observers.remove(o); }
public void sendMessage(String message) {
this.message = message;
}
public void notifyListeners() {
for(MessageListener o : observers) { // Look Here
if (message.contains("order")) {
o.onMessageReceived("new order");
}
if (message.contains("disconnect")) {
o.onDisconnect();
}
}
}
}
/**
Client received message: new order
Client disconnected
**/
Last update: 267 days ago