We all know the three basic principles of OOP: Encapsulation, Inheritance and Polymorphism. And there is also this fourth principle: Data Abstraction; though it’s not always mentioned as a standalone principle, as it is closely tied with encapsulation. Today I am going to discuss a simple case to display the power and necessity of Inheritance.
Let’s assume a scenario: you are working on an application, which has to perform a server call asynchronously and has no direct impact on the UI. But when the server returns a response, you have to make some modification to your application regardless of the present UI.
Now on a detailed note; you have three UIs represented by class A, B and C. These are on a navigation stack: A being the root class (the first one to be displayed) and class B and C are pushed on the stack depending on user interaction (top class from the stack is available for user interaction). When the application launches with class A being available for user interaction, a server call is made asynchronously with the users location (latitude and longitude) and the server will respond with an array of nearby attractions that may interest the user. Now, here is the catch: you have to catch the response regardless of the class currently available for user interaction (top of the stack!); i.e. it does not matter if class A, B or C is on top of the stack (in memory!); you have to catch the response, format it and display it to the current screen! Now, how do you do that?
Well there are a lot of possible ways, but using inheritance might be the easiest and most convenient one. How do we do that? Remember the definition:
Objects can relate to each other with either a “has a”, “uses a” or an “is a” relationship.
“Is a” is the inheritance way of object relationship. Now if we can make sure class A, B and C all are a class S object, we make sure it is always in the memory and on top of the stack! Right?
So, we create a super class S and write two methods:
1. public void makeServerCall()
2. public void serverCallWithResponse (bool success, String[] nearbyLocations, Error err)
And we make class A, B and C subclass of class S. This makes sure, the public methods 1 and 2 are accessible and overridable by all of them (class A, B and C).
Now from class A call method 1 to make the server call and in all three classes (A, B and C) override the method 2. This makes sure, regardless of the present UI we receive the server response and act accordingly, hence solve the problem!
I really hope this helps someone, and I am planning to write more on this kind of little things! Any suggestions, better approach or any criticism is most welcome and appreciated! 🙂
Cheers!