# Observer

Paul Tomoiu

●

Examples,

Design patterns

●

Published 2022-12-03

This tutorial explains to you the design pattern named Observer (which is a Structural Pattern).

# Observer Pattern - theory

Observer Design Pattern in Java is used when we want to have a one-to-many dependency and when one object change a state ("salary" value in Employee class, in my example) all its dependents are notified and updated automatically. The Observer pattern is also known as Publish-Subscribe or Dependents.

Take a look at the following UML diagram representing the Observer design pattern (for my example):

# Observer Pattern - example

Here is an example of using the Observer Design Pattern in Java:

package observer.java.pattern.example;
 
import java.util.ArrayList;
import java.util.List;
 
public class Employee {
 
    int id;
    String name; 
    int salary;
     
    private List<Observer> observers = new ArrayList<Observer>();
     
    public Employee(int id, String name, int salary) {
        this.id = id;
        this.name = name;
        this.salary = salary;
    }
 
    public String getName() {
        return name;
    }
     
    public int getSalary() {
        return salary;
    }
     
    public void setName(String name) {
        this.name = name;
    }
     
    public void setSalary(int salary) {
        this.salary = salary;
        notifyAllObservers();
    }
     
    public void notifyAllObservers(){
          for (Observer observer : observers) {
             observer.sendMessage();
          }
    }
     
    public void addObserver(Observer observer){
          observers.add(observer);      
    }
 
}

package observer.java.pattern.example;
 
public interface Observer {
 
    public abstract void sendMessage();
}

package observer.java.pattern.example;
 
public class HumanResourcesObs implements Observer {
 
    private Employee emp;
     
    public HumanResourcesObs(Employee employee){
         
        this.emp = employee;        
        this.emp.addObserver(this);     
    }
 
    @Override
    public void sendMessage() {
         System.out.println( "Human Resources are informed about the new salary ("+ emp.getSalary()+ ") of Mr. "+emp.getName() );
    }
}

package observer.java.pattern.example;
 
public class HighManagementObs implements Observer {
 
    private Employee emp;
     
    public HighManagementObs (Employee employee){
         
        this.emp = employee;            
        this.emp.addObserver(this); 
    }
 
    @Override
    public void sendMessage() {
         System.out.println( "High Management is informed about the new salary ("+ emp.getSalary()+ ") of Mr. "+emp.getName() );
    }
}

package observer.java.pattern.example;
 
public class ObserverJavaPatternExample {
 
    public static void main(String[] args) {
         
        Employee emp = new Employee(10, "John Brown", 2000);
 
        new HumanResourcesObs(emp);
        new HighManagementObs(emp);
         
        emp.setSalary(2200);
        System.out.println("-------------------------------------------------");
        emp.setSalary(3300);
    }
}

When you run this example you will receive the following result: