Observer Pattern (in JAVA)

 

Definition-define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.

Advantage:

1.       loosely coupled designs

when two objects are loosely coupled, they can interact, but have very little knowledge of each other. The only thing the subject knows about an observer is that it implements a certain interface. And also, we can add new observers at any time. Changes to either the subject or an observer will no affect the other.

2.       Problem oriented

a.       for one-to-many relationship

b.       many objects wants the calls from the subject, which is always described as dependency.

 

Example coded in JAVA:

//Subject.java

  

import java.util.*;
public interface Subject ...{
    public void registerObserver(Observer o);
    public void removeObserver(Observer o);
    public void notifyObservers();
}

 

 //Observer.java

public interface Observer ...{
    public void update(String Career,String Knowledge);
}

//InfoCenter.java 

//this is the concrete class of Subject

 

public class InfoCenter implements Subject ...{

    ArrayList observers;
    String CareerInfo;
    String KnowledgeInfo;
    
    public InfoCenter()
    ...{
        observers=new ArrayList<Observer>();
    }
    public void notifyObservers() ...{
        
        for(int i=0;i<observers.size();i++)
        ...{
            Observer o=(Observer)observers.get(i);
            o.update(this.CareerInfo,this.KnowledgeInfo);
        }
    }
    public void SetNewInfomation(String CareerInfo,String KnowledgeInfo)
    ...{
        this.CareerInfo = CareerInfo;
        this.KnowledgeInfo = KnowledgeInfo;
        notifyObservers();
    }
    
    public void registerObserver(Observer o) ...{
        // TODO Auto-generated method stub
        observers.add(o);
    }

    @Override
    public void removeObserver(Observer o) ...{
        // TODO Auto-generated method stub
        int i=observers.indexOf(o);
        if(i>=0)
        ...{
            observers.remove(i);
        }
    }
    public String GetCareerInfo()
    ...{
        return this.CareerInfo;
    }
    public String GetKnowledgeInfo()
    ...{
        return this.KnowledgeInfo;
    }

}

//PeopleSubscrie.java

//this is the concrete class of observer

 

public class PeopleSubscribe implements Observer ...{

    Subject sub;
    private String CareerInfo;
    public PeopleSubscribe(Subject sub)
    ...{
        this.sub=sub;
        this.sub.registerObserver(this);
    }
    
    public void update(String Career,String Knowledge) ...{
        // TODO Auto-generated method stub
        this.CareerInfo = Career;
        Display();
    }
    private void Display()
    ...{
        System.out.println("I've subscribed the career infomation,current career info is:");
        System.out.println(CareerInfo);
    }

}

//Main.java

//codes executed

 

public class Main ...{

    public static void main(String[] args) ...{
        // TODO Auto-generated method stub
        InfoCenter c=new InfoCenter();
        
        Observer people1=new PeopleSubscribe(c);//subscribe the career information
        Observer people2=new PeopleSubscribe(c);//subscribe the career information
        
        c.SetNewInfomation("NJ Software Designer", "JDK");
        
        c.removeObserver(people1); //remove observer
        
        c.SetNewInfomation("BJ System Analysist", "AJAX in Java");
    }

}