class TestDvdObserver { public static void main(String[] args) { DvdReleaseByCategory btvs = new DvdReleaseByCategory("Buffy the Vampire Slayer"); DvdReleaseByCategory simpsons = new DvdReleaseByCategory("The Simpsons"); DvdReleaseByCategory sopranos = new DvdReleaseByCategory("The Sopranos"); DvdReleaseByCategory xfiles = new DvdReleaseByCategory("The X-Files"); DvdSubscriber jsopra = new DvdSubscriber("Junior Soprano"); DvdSubscriber msimps = new DvdSubscriber("Maggie Simpson"); DvdSubscriber rgiles = new DvdSubscriber("Rupert Giles"); DvdSubscriber smulde = new DvdSubscriber("Samantha Mulder"); DvdSubscriber wrosen = new DvdSubscriber("Willow Rosenberg"); btvs.addSubscriber(rgiles); btvs.addSubscriber(wrosen); simpsons.addSubscriber(msimps); sopranos.addSubscriber(jsopra); xfiles.addSubscriber(smulde); xfiles.addSubscriber(wrosen); DvdRelease btvsS2 = new DvdRelease("DVDFOXBTVSS20", "Buffy The Vampire Slayer Season 2", 2002, 06, 11); DvdRelease simpS2 = new DvdRelease("DVDFOXSIMPSO2", "The Simpsons Season 2", 2002, 07, 9); DvdRelease soprS2 = new DvdRelease("DVDHBOSOPRAS2", "The Sopranos Season 2", 2001, 11, 6); DvdRelease xfilS5 = new DvdRelease("DVDFOXXFILES5", "The X-Files Season 5", 2002, 04, 1); btvs.newDvdRelease(btvsS2); simpsons.newDvdRelease(simpS2); sopranos.newDvdRelease(soprS2); xfiles.newDvdRelease(xfilS5); xfiles.removeSubscriber(wrosen); xfilS5.updateDvdReleaseDate(2002, 5, 14); xfiles.updateDvd(xfilS5); } }

The above code was from http://www.higherpass.com/javapatterns/JavaDesignPatternsObserver/java-observer.php. This code shows the observer pattern in java. The DvdReleaseByCategory is the subject and DvdSubscriber is the observer. In this example it is used to keep track of new dvd releases. When someone creates a new DvdReleaseByCategory object they are basically creating a dvd category. For example, if one were to create the object, btvs, they would be creating a tv category of Buffy the Vampire Slayer. Now if someone wanted to become a subscriber to that category they would need to use the addSubscriber method. The addSubscriber method takes an object of type DvdSubscriber. Once the subscriber has been added, whenever there is a new DvdRelease, that subscriber will be notified.