When should we use Observer and Observable

Question

An interviewer asked me   What is Observer and Observable and when should we use them   I wasn t aware of these terms  so when I got back home and started Googling about Observer and Observable  I found some points from different resources      1  Observable is a class and Observer is an interface       2  The Observable class maintains a list of Observers       3  When an Observable object is updated  it invokes the update   method of each of its Observers to notify that  it is changed    I found this example   import java util Observable  import java util Observer   class MessageBoard extends Observable       public void changeMessage String message                 setChanged            notifyObservers message            class Student implements Observer         Override     public void update Observable o  Object arg                 System out println  Message board changed      arg            public class MessageBoardTest        public static void main String   args                 MessageBoard board   new MessageBoard            Student bob   new Student            Student joe   new Student            board addObserver bob           board addObserver joe           board changeMessage  More Homework               But I don t understand why we need Observer and Observable  What are the setChanged   and notifyObservers message  methods for

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Observer a k a callback is registered at Observable   It is used for informing e g  about events that happened at some point of time  It is widely used in Swing  Ajax  GWT for dispatching operations on e g  UI events  button clicks  textfields changed etc    In Swing you find methods like addXXXListener Listener l   in GWT you have  Async callbacks   As list of observers is dynamic  observers can register and unregister during runtime  It is also a good way do decouple observable from observers  as interfaces are used

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In very simple terms  because the other answers are referring you to all the official design patterns anyway  so look at them for further details    If you want to have a class which is monitored by other classes in the ecosystem of your program you say that you want the class to be observable  I e  there might be some changes in its state which you would want to broadcast to the rest of the program   Now  to do this we have to call some kind of method  We don t want the Observable class to be tightly coupled with the classes that are interested in observing it  It doesn t care who it is as long as it fulfils certain criteria   Imagine it is a radio station  it doesn t care who is listening as long as they have an FM radio tuned on their frequency   To achieve that we use an interface  referred to as the Observer    Therefore  the Observable class will have a list of Observers  i e  instances implementing the Observer interface methods you might have   Whenever it wants to broadcast something  it just calls the method on all the observers  one after the other   The last thing to close the puzzle is how will the Observable class know who is interested  So the Observable class must offer some mechanism to allow Observers to register their interest  A method such as addObserver Observer o  internally adds the Observer to the list of observers  so that when something important happens  it loops through the list and calls the respective notification method of the Observer interface of each instance in the list   It might be that in the interview they did not ask you explicitly about the java util Observer and java util Observable but about the generic concept  The concept is a design pattern  which Java happens to provide support for directly out of the box to help you implement it quickly when you need it  So I would suggest that you understand the concept rather than the actual methods classes  which you can look up when you need them    UPDATE  In response to your comment  the actual java util Observable class offers the following facilities    Maintaining a list of java util Observer instances  New instances interested in being notified can be added through addObserver Observer o   and removed through deleteObserver Observer o   Maintaining an internal state  specifying whether the object has changed since the last notification to the observers  This is useful because it separates the part where you say  that the Observable has changed  from the part where you notify the changes   E g  Its useful if you have multiple changes happening and you only want to notify at the end of the process rather than at each small step   This is done through setChanged    So you just call it when you changed something to the Observable and you want the rest of the Observers to eventually know about it  Notifying all observers that the specific Observable has changed state  This is done through notifyObservers    This checks if the object has actually changed  i e  a call to setChanged   was made  before proceeding with the notification  There are 2 versions  one with no arguments and one with an Object argument  in case you want to pass some extra information with the notification  Internally what happens is that it just iterates through the list of Observer instances and calls the update Observable o  Object arg  method for each of them  This tells the Observer which was the Observable object that changed  you could be observing more than one   and the extra Object arg to potentially carry some extra information  passed through notifyObservers

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I tried to figure out  why exactly we need Observer and Observable   As previous answers already stated  they provide means of subscribing an observer to receive automatic notifications of an observable    One example application where this may be useful is in data binding  let s say you have some UI that edits some data  and you want the UI to react when the data is updated  you can make your data observable  and subscribe your UI components to the data  Knockout js is a MVVM javascript framework that has a great getting started tutorial  to see more observables in action I really recommend going through the tutorial  http   learn knockoutjs com   I also found this article in Visual Studio 2008 start page  The Observer Pattern is the foundation of Model View Controller  MVC  development  http   visualstudiomagazine com articles 2013 08 14 the-observer-pattern-in-net aspx

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Since Java9  both interfaces are deprecated  meaning you should not use them anymore  See Observer is deprecated in Java 9  What should we use instead of it   However  you might still get interview questions about them

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Definition  Observer pattern is used when there is one to many relationship between objects such as if one object is modified  its dependent objects are to be notified automatically and corresponding changes are done to all dependent objects   Examples   Let s say  your permanent address is changed then you need to notify passport authority and pan card authority  So here passport authority and pan card authority are observers and You are a subject  On Facebook also  If you subscribe to someone then whenever new updates happen then you will be notified    When to use it    When one object changes its state  then all other dependents object must automatically change their state to maintain consistency When the subject doesn t know about the number of observers it has  When an object should be able to notify other objects without knowing who objects are    Step 1  Create Subject class   Subject java    import java util ArrayList    import java util List     public class Subject      private List lt Observer gt  observers            new ArrayList lt Observer gt       private int state     public int getState         return state        public void setState int state       this state   state     notifyAllObservers           public void attach Observer observer        observers add observer                  public void notifyAllObservers        for  Observer observer   observers         observer update                  Step 2  Create Observer class   Observer java  public abstract class Observer      protected Subject subject     public abstract void update        Step 3  Create concrete observer classes  BinaryObserver java  public class BinaryObserver extends Observer     public BinaryObserver Subject subject        this subject   subject       this subject attach this           Override   public void update          System out println   Binary String            Integer toBinaryString  subject getState                  OctalObserver java  public class OctalObserver extends Observer      public OctalObserver Subject subject        this subject   subject      this subject attach this          Override   public void update         System out println   Octal String           Integer toOctalString  subject getState                  HexaObserver java  public class HexaObserver extends Observer     public HexaObserver Subject subject       this subject   subject      this subject attach this          Override   public void update          System out println   Hex String           Integer toHexString  subject getState     toUpperCase              Step 4  Use Subject and concrete observer objects   ObserverPatternDemo java   public class ObserverPatternDemo       public static void main String   args           Subject subject   new Subject            new HexaObserver subject          new OctalObserver subject          new BinaryObserver subject           System out println  First state change  15               subject setState 15          System out println  Second state change  10              subject setState 10           Step 5  Verify the output   First state change  15  Hex String  F  Octal String  17  Binary String  1111  Second state change  10  Hex String  A  Octal String  12  Binary String  1010

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Observer pattern is used when there is one-to-many relationship between objects such as if one object is modified  its dependent objects are to be notified automatically

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They are parts of the Observer design pattern  Usually one or more obervers get informed about changes in one observable  It s a notifcation that  something  happened  where you as a programmer can define what  something  means   When using this pattern  you decouple the both entities from each another - the observers become pluggable

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I have written a short description of the observer pattern here  http   www devcodenote com 2015 04 design-patterns-observer-pattern html  A snippet from the post   Observer Pattern   It essentially establishes a one-to-many relationship between objects and has a loosely coupled design between interdependent objects   TextBook Definition  The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state  all of its dependents are notified and updated automatically   Consider a feed notification service for example  Subscription models are the best to understand the observer pattern

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If the interviewer asks to implement Observer design pattern without using Observer classes and interfaces  you can use the following simple example    MyObserver as observer interface  interface MyObserver        void update MyObservable o  Object arg       MyObservable as Observable class  class MyObservable       ArrayList lt MyObserver gt  myObserverList   new ArrayList lt MyObserver gt          boolean changeFlag   false       public void notifyObservers Object o                if  hasChanged                          for MyObserver mo   myObserverList                    mo update this  o                             clearChanged                          public void addObserver MyObserver o            myObserverList add o                      public void setChanged             changeFlag   true             public boolean hasChanged             return changeFlag             protected void clearChanged             changeFlag   false                        Your example with MyObserver and MyObservable   class MessageBoard extends MyObservable     private String message     public String getMessage         return message         public void changeMessage String message        this message   message      setChanged        notifyObservers message          public static void main String   args        MessageBoard board   new MessageBoard        Student bob   new Student        Student joe   new Student        board addObserver bob       board addObserver joe       board changeMessage  More Homework            class Student implements MyObserver       Override   public void update MyObservable o  Object arg        System out println  Message board changed      arg

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You have a concrete example of a Student and a MessageBoard   The Student registers by adding itself to the list of Observers that want to be notified when a new Message is posted to the MessageBoard   When a Message is added to the MessageBoard  it iterates over its list of Observers and notifies them that the event occurred   Think Twitter   When you say you want to follow someone  Twitter adds you to their follower list   When they sent a new tweet in  you see it in your input   In that case  your Twitter account is the Observer and the person you re following is the Observable   The analogy might not be perfect  because Twitter is more likely to be a Mediator   But it illustrates the point

[java] When should we use Observer and Observable?

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