2. Observer mode

Observer pattern Definition: defines a one-to-many dependency between objects, so that when an object state changes, all its dependents are notified and updated automatically. Personal understanding: When the subject changes, the changes are notified in a timely manner to each observer (subscriber) Observer pattern design method that subscribes to the subject, allowing for a loose coupling between the subject and the observer, adapting to change.

• The Observer pattern defines a one-to-many relationship between objects
• Topic using a common interface to update the Observer
• A loosely coupled approach between the subject and the Observer
• Subject can be used by the observer mode with push or pull data
• Cannot rely on a specific notification order when there are multiple observers
• Java has a variety of observer patterns, such as universal java.util.Observable
• Swing uses observer patterns heavily
• This mode is also used in JavaBeans, RMI applications.

Define the topic interface, provide a topic must implement three methods: registration, deletion, notification

PublicInterface  Subject {    void  registerobserver (Observer Observer);     void removeobserver (Observer Observer);     void notifyobservers ();}

Achieve the specific weather theme: Implement subject interface, implement registration, deletion, notification method; Define the Observer collection used for observer registration; Define the Setmeasurements method for updating weather data. The Measurementschanaged method is called by default within the method (call Notifyobservers, loop Observer collection, call Update method for each observer)

 Public classWeatherdatasubjectImplementssubject{List<Observer>observers; Private floattemp; Private floathumidity; Private floatpressure;  PublicWeatherdatasubject () {observers=NewArraylist<>(); }     Public voidregisterobserver (Observer Observer) {observers.add (Observer); }     Public voidremoveobserver (Observer Observer) {observers.remove (Observer); }     Public voidnotifyobservers () { for(Observer observer:observers) {observer.update (temp, humidity, pressure); }    }     Public voidmeasurementschanaged () {notifyobservers (); }     Public voidSetmeasurements (floatTempfloatHumidity,floatpressure) {         This. temp =temp;  This. Humidity =humidity;  This. Pressure =pressure;    Measurementschanaged (); }}

Define the Observer interface: The Update method that each observer must implement to be called by the theme to implement the notification function

 Public Interface Observer {    void update (float temp,float humidity,float  pressure);}

Define the viewer display method:

 Public Interface displayelement {    void  display ();}

The following three observers implement the observer interface, the Displayelement display interface: In the Update method, the display method is called by default, and the personalized display mode is defined in the specific observer.

 Public classCurrentconditionsdisplayImplementsobserver,displayelement {Private floattemp; Private floathumidity; Private floatpressure; @Override Public voiddisplay () {System.out.println ("Currentconditionsdisplay:temp=" +temp+ "humidity=" +humidity+ ";p ressure=" +pressure); } @Override Public voidUpdatefloatTempfloatHumidity,floatpressure) {         This. temp =temp;  This. Humidity =humidity;  This. Pressure =pressure;    Display (); }} Public classStatisticsdisplayImplementsobserver,displayelement {Private floattemp; Private floathumidity; Private floatpressure; @Override Public voiddisplay () {System.out.println ("Statisticsdisplay:temp=" +temp+ "humidity=" +humidity+ ";p ressure=" +pressure); } @Override Public voidUpdatefloatTempfloatHumidity,floatpressure) {         This. temp =temp;  This. Humidity =humidity;  This. Pressure =pressure;    Display (); }} Public classThirdpartydisplayImplementsobserver,displayelement {Private floattemp; Private floathumidity; Private floatpressure; @Override Public voiddisplay () {System.out.println ("Thirdpartydisplay:temp=" +temp+ "humidity=" +humidity+ ";p ressure=" +pressure); } @Override Public voidUpdatefloatTempfloatHumidity,floatpressure) {         This. temp =temp;  This. Humidity =humidity;  This. Pressure =pressure;    Display (); }}

Test class:

 Public classTest { Public Static voidMain (string[] args) {//observersCurrentconditionsdisplay Currentconditionsdisplay =NewCurrentconditionsdisplay (); Statisticsdisplay Statisticsdisplay=NewStatisticsdisplay (); Thirdpartydisplay Thirdpartydisplay=NewThirdpartydisplay (); //ThemeWeatherdatasubject subject =NewWeatherdatasubject (); //registering the viewer with the subjectSubject.registerobserver (Currentconditionsdisplay);        Subject.registerobserver (Statisticsdisplay);        Subject.registerobserver (Thirdpartydisplay); Subject.setmeasurements (1,1,1); Subject.setmeasurements (2,3,4); }}

Output Result:

currentconditionsdisplay:temp=1.0;humidity=1.0;pressure=1.0
statisticsdisplay:temp=1.0;humidity=1.0;pressure=1.0
thirdpartydisplay:temp=1.0;humidity=1.0;pressure=1.0

currentconditionsdisplay:temp=2.0;humidity=3.0;pressure=4.0
statisticsdisplay:temp=2.0;humidity=3.0;pressure=4.0
thirdpartydisplay:temp=2.0;humidity=3.0;pressure=4.0

2. Observer mode