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POJava Syntax: Who can an interface be an object?
 

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    <p>I'm currently studying the Observer pattern and I've come across some confusion. </p> <p>When I read this line of code:</p> <pre><code>IObserverSubscribe user1= new ConcreteObserverYoutubeUser(); </code></pre> <p>I think the interface <code>IObserverSubscribe user1</code> is creating and instantiating <code>new ConcreteObserverYoutubeUser()</code>. That is a little confusing to me because usually the same class that is being declared is also instantiating. Should it be like this:</p> <pre><code>IObserverSubscribe user1= new IObserverSubscribe(); </code></pre> <p>Why is the interface decoration able to instantiate a another class?</p> <p>Full code below:</p> <p>Main:</p> <pre><code>package observerpattern; public class ObserverPattern { /** * The observer pattern is a software design pattern in which * an object, called the subject, maintains a list of its dependents, * called observers, and notifies them automatically of any * state changes, usually by calling one of their methods. * It is mainly used to implement distributed event handling systems. * The Observer pattern is also a key part in the familiar * Model View Controller (MVC) architectural pattern. */ public static void main(String[] args) { SubjectYouTubeChannel sytc= new SubjectYouTubeChannel();// create youtube channel IObserverSubscribe user1= new ConcreteObserverYoutubeUser(); IObserverSubscribe user2= new ConcreteObserverYoutubeUser(); IObserverSubscribe moderator1= new ConcreteObserverYoutubeModerator(); sytc.Subscribe(user1); sytc.Subscribe(user2); sytc.Subscribe(moderator1); sytc.Unsubscribe(user2); sytc.notifySubscribers(); } } </code></pre> <p>subject:</p> <pre><code>package observerpattern; import java.util.ArrayList; import java.util.List; import observerpattern.IObserverSubscribe; public class SubjectYouTubeChannel { private List&lt;IObserverSubscribe&gt; subscribers = new ArrayList&lt;IObserverSubscribe&gt;(); public void Subscribe(IObserverSubscribe ios){ subscribers.add(ios); } public void Unsubscribe(IObserverSubscribe ios){ subscribers.remove(ios); } public void notifySubscribers(){ for(IObserverSubscribe ios : subscribers ){ ios.Notify(); } } } </code></pre> <p>Interface Observer:</p> <pre><code>package observerpattern; public interface IObserverSubscribe { void Notify(); } </code></pre> <p>Concrete Observer:</p> <pre><code>package observerpattern; public class ConcreteObserverYoutubeUser implements IObserverSubscribe{ @Override public void Notify() { System.out.println("User watches video, comments, ect"); } } </code></pre>
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