Observer pattern is used when there isone to many relationship between objects such as if one object is modified, its dependent objects are to be notified automatically. Observer pattern falls under behavioural pattern category.
Example: a Youtube Channel and its subscribers
It is also called pub sub model
Whenever new contents in the youtube channel is added or existing content is modified, all the subscribers of the youtube channel will be notified
Let's understand the same example
Subject orYoutubeChannel
packagePatterns.Behavioral.observer;importjava.util.ArrayList;importjava.util.List;publicclassYouTubeChannelSubject{privateStringchannelName;privateList<String>contents;List<Observer>subscriberObservers;publicYouTubeChannelSubject(){this.channelName="Mr.Beast";contents=newArrayList<>();subscriberObservers=newArrayList<>();}publicvoidaddNewContent(Stringname){contents.add(name);//notify all listenersnotifySubscriberObserver();}publicvoidremoveContent(Stringname){if(contents.contains(name)){contents.remove(name);//notify all listenersnotifySubscriberObserver();}}publicList<String>getContents(){returnthis.contents;}publicStringgetChannelName(){returnthis.channelName;}publicvoidaddObserver(Observerobserver){this.subscriberObservers.add(observer);}publicvoidremoveObserver(Observerobserver){this.subscriberObservers.remove(observer);}publicvoidnotifySubscriberObserver(){for(Observersubscriber:subscriberObservers){subscriber.update();}}}Abstract Subscriber orObserver
packagePatterns.Behavioral.observer;publicabstractclassObserver{publicStringsubscriberName;publicYouTubeChannelSubjectsubject;publicObserver(YouTubeChannelSubjectyouTubeChannelSubject,Stringname){this.subject=youTubeChannelSubject;this.subscriberName=name;}abstractvoidupdate();publicYouTubeChannelSubjectgetSubject(){returnthis.subject;}}Concrete Subscribers
packagePatterns.Behavioral.observer;publicclassSubscriberOneObserverextendsObserver{publicSubscriberOneObserver(YouTubeChannelSubjectsubject,Stringname){super(subject,name);// add this subscriber as of the observers of the youtube channel defined by object 'subject'this.subject.addObserver(this);}@Overridevoidupdate(){System.out.println(this.subscriberName+" got updated !");System.out.println(this.subject.getChannelName()+" has updated contents, the total content on the channel now is "+this.subject.getContents().size());System.out.println("List of contents on the channel "+this.subject.getChannelName()+" are as follows: ");for(Stringcontent:subject.getContents()){System.out.println(content);}System.out.println("------------------------------");}}packagePatterns.Behavioral.observer;publicclassSubscriberTwoObserverextendsObserver{publicSubscriberTwoObserver(YouTubeChannelSubjectsubject,Stringname){super(subject,name);// add this subscriber as of the observers of the youtube channel defined by object 'subject'this.subject.addObserver(this);}@Overridevoidupdate(){System.out.println(this.subscriberName+" got updated !");System.out.println(this.subject.getChannelName()+" has updated contents, the total content on the channel now is "+this.subject.getContents().size());System.out.println("List of contents on the channel "+this.subject.getChannelName()+" are as follows: ");for(Stringcontent:subject.getContents()){System.out.println(content);}System.out.println("------------------------------");}}Main
packagePatterns.Behavioral.observer;publicclassMain{publicstaticvoidmain(Stringargs[]){//creating youtube channelYouTubeChannelSubjectsubject=newYouTubeChannelSubject();//subscriber's of the above youtube channelnewSubscriberOneObserver(subject,"prashant");newSubscriberTwoObserver(subject,"sandeep");//now updating the contents of the youtube channelsubject.addNewContent("Men Vs Women survive the wilderness of $500,000");subject.addNewContent("7 Days stranded in a cave");}}output
prashant got updated !Mr.Beast has updated contents, the total content on the channel now is 1List of contents on the channel Mr.Beast are as follows: Men Vs Women survive the wilderness of $500,000------------------------------sandeep got updated !Mr.Beast has updated contents, the total content on the channel now is 1List of contents on the channel Mr.Beast are as follows: Men Vs Women survive the wilderness of $500,000------------------------------prashant got updated !Mr.Beast has updated contents, the total content on the channel now is 2List of contents on the channel Mr.Beast are as follows: Men Vs Women survive the wilderness of $500,0007 Days stranded in a cave------------------------------sandeep got updated !Mr.Beast has updated contents, the total content on the channel now is 2List of contents on the channel Mr.Beast are as follows: Men Vs Women survive the wilderness of $500,0007 Days stranded in a cave------------------------------prashantmishra@Prashants-MacBook-Air machinecoding %Top comments(0)
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