Behavioral – Observer Design Pattern

August 6, 2020 2 min read

Observer pattern is classified among Behavioral patterns. This pattern is using if there is a many-to-many relationship between the objects inside a program. That means when a behavior of one object is affecting to many other objects inside the program.

Let’s check it out from an example in java.

This is on a salary calculation between managers and employees.

Here I created a class called “Salary” first and made a list to add observers to the list. Because “Salary” is the main object that keeps the observers with it. But to add those observers I should make “Observer” class.

After creating the observer class, let’s go again to the “Salary” class. There you can see the method “attach()”. From this method I can add observers to the list. Then I created another variable called “value” to keep the real salary and by including the getters and setters I made it as a property.

You can see I called a method called “notifyAllObervers()” method inside the “getValue()” method. “notifyAllOberver” method is created here to iterate the observers as we know for each observer “update()” method is given by the class “Observer”.

Here I created 2 classes called “ManagerBonus” and “EmployeeBonus” by extending from the abstract class “Observer” as they are the observer classes that observe the main object. If we change the salary value in any time, those 2 classes update their salaries by the “update()” method that is overridden. This is the importance of using observer design pattern. Let’s go to the main method.