The observer pattern for Java design patterns

Observer pattern: Defines a one-to-many dependency between objects. When the state of an object changes, all objects that depend on it are notified and automatically updated.

A simple example: for example, there are two people Zhang San and John Doe subscribed to weather messages from the meteorological observatory.

1  Public classSubject2 {3     PrivateList<observer> observers =NewArraylist<observer>();4 5      Public voidAttach (Observer Observer)6     {7 OBSERVERS.ADD (Observer);8Observer.update ( This);9     }Ten      One      Public voidDetach (Observer Observer) A     { - OBSERVERS.REMOVE (Observer); -     } the      -     protected voidnotifyobservers () -     { -          for(Observer observer:observers) +         { -Observer.update ( This); +         } A     } at } -  -  Public InterfaceObserver - { -      Public voidUpdate (Subject Subject); - } in  -  Public classConcreteSubjectextendsSubject to { +     PrivateString subjectstate; -  the      PublicString getsubjectstate () *     { $         returnsubjectstate;Panax Notoginseng     } -  the      Public voidsetsubjectstate (String subjectstate) +     { A          This. subjectstate =subjectstate; the          This. Notifyobservers (); +     } - } $  $ Import Staticjava.lang.System.out; -  Public classConcreteobserverImplementsObserver - { the     PrivateString observerstate; - Wuyi      Public voidUpdate (Subject Subject) the     { -Observerstate =((ConcreteSubject) subject). Getsubjectstate (); Wu     } -      About      Public voidinfo () $     { -Out.println ("I am:" +observername); -Out.println ("Info:" +observerstate); - out.println (); A     } +      the     PrivateString Observername; -  $      Public voidsetobservername (String observername) the     { the          This. Observername =Observername; the     } the } -  in  Public classClient the { the      Public Static voidMain (string[] args) About     { theConcreteSubject Weathersubject =NewConcreteSubject (); theWeathersubject.setsubjectstate ("Sunny, 25°"); the          +Concreteobserver Zhangsan =Newconcreteobserver (); -Zhangsan.setobservername ("Zhang San"); the         BayiConcreteobserver Lisi =Newconcreteobserver (); theLisi.setobservername ("John Doe"); the          - Weathersubject.attach (Zhangsan); - Weathersubject.attach (Lisi); the          the zhangsan.info (); the lisi.info (); the          - Weathersubject.detach (Zhangsan); theWeathersubject.setsubjectstate ("Cloudy, 20°"); the          the zhangsan.info ();94 lisi.info (); the     } the } the 98 I am: Zhang San AboutInfo: Sunny, 25° - 101 I am: John Doe102Info: Sunny, 25°103 104 I am: Zhang San theInfo: Sunny, 25°106 107 I am: John Doe108Info: Cloudy, 20°

The observer pattern for Java design patterns