Observer

Story
Image
UML
Implementation
Usage

Story

Keep me updated.

Newslettter subscription demonstrate Observer pattern. A newsletter is a regularly distributed publication that is generally about one main topic of interest to its subscribers. Subscribers can subscribe or unsubscribe to the newsletters.

Image

alt text

UML

Implementation

Observer.java

package com.hundredwordsgof.observer;

/**
 * Observer defines an updating interface for objects that should be notified of
 * changes in a subject.
 *
 */
public interface Observer {

  void update();
}

ConcreteObserver.java

package com.hundredwordsgof.observer;

/**
 * ConcreteObserver maintains a reference to a ConcreteSubject object, stores
 * state that should stay consistent with the subject's, implements the Observer
 * updating interface to keep its state consistent with the subject's.
 *
 */
public class ConcreteObserver implements Observer {

  private int observerState;

  private ConcreteSubject subject;

  public ConcreteObserver(ConcreteSubject subject) {
    this.subject = subject;
  }

  public void update() {
    observerState = subject.getState();
  }

  protected int getObserverState() {
    return observerState;
  }
}

Subject.java

package com.hundredwordsgof.observer;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

/**
 * Subject knows its observers. Any number of Observer objects may observe a
 * subject.
 *
 */
abstract class Subject {

  private List<Observer> observers = new ArrayList<Observer>();

  public void attach(Observer observer) {
    observers.add(observer);
  }

  public void dettach(Observer observer) {
    observers.remove(observer);
  }

  public void notifyObervers() {
    for (Iterator iterator = observers.iterator(); iterator.hasNext();) {
      Observer observer = (Observer) iterator.next();
      observer.update();
    }
  }
}

ConcreteSubject.java

package com.hundredwordsgof.observer;

/**
 * ConcreteSubject stores state of interest to ConcreteObserver objects, sends a
 * notification to its observers when its state changes.
 *
 */
public class ConcreteSubject extends Subject {

  private int state;

  public int getState() {
    return state;
  }

  public void setState(int state) {
    this.state = state;
    this.notifyObervers();
  }
}

Usage

ObserverTest.java

package com.hundredwordsgof.observer;

import static org.junit.Assert.assertEquals;
import org.junit.Test;

/**
 * Test Observer pattern.
 */
public class ObserverTest {

  @Test
  public void testObserver() throws CloneNotSupportedException {

    ConcreteSubject subject = new ConcreteSubject();

    Observer observer = new ConcreteObserver(subject);
    subject.attach(observer);
    subject.setState(1);

    // changes via subject.setState is propagated towards observer
    assertEquals(1, ((ConcreteObserver) observer).getObserverState());

    subject.dettach(observer);
    subject.setState(0);

    // observer is detached so changes are not propageted
    assertEquals(1, ((ConcreteObserver) observer).getObserverState());
  }
}