Open / Closed Principle

#solid #java #ocp

The open closed principle is the O in the SOLID principles. It was created by Bertrand Meyer and states:

Software entities should be open for extension, but closed for modification

Let's dive deeper with an example.

We've been tasked with creating a platform that merges all video streaming sites. The platform will be able to add a platform to its list of platforms, search for a video on all platforms and play a video from the search results. The class can look like so:

	interface Video{
	public void play();
	}

	class Netflix{
	public Video search(String searchQuery){
	//.. implement search function
	}
	}

	class Hulu{
	public Video search(String searchQuery){
	//.. implement search function
	}
	}

	class AmazonVideo{
	public Video search(String searchQuery){
	//.. implement search function
	}
	}

	class Hbo{
	public Video search(String searchQuery){
	//.. implement search function
	}
	}

	class CentralVideoStreamingPlatform{

	public void add(Netflix netflix){}
	public void add(Hulu hulu){}
	public void add(AmazonVideo amazonVideo){}
	public void add(Hbo hbo){}

	public Video searchVideo(String searchQuery){
	//.. implement search fucntion
	}

	public void play(Video selectedVideo){}

	private ArrayList<VideoStreamingPlatform> platforms = new ArrayList<VideoStreamingPlatform>();
	}

view raw ocp_example1.java hosted with ❤ by GitHub

CentralVideoStreamingPlatform covers most mainstream video streaming platforms. However, if we were to add another platform, say DisneyPlus, another #add() would be created. This directly violates the open for extension aspect of OCP.CentralVideoStreamingPlatform must be open to accept different details without needing to change code that's already written. By overloading #add() again, we are needlessly changing CentralVideoStreamingPlatform. Who's to say how many #add() CentralVideoStreamingPlatform would have as more streaming platforms are added?

To ensure CentralVideoStreamingPlatform's extensibility, we can refactor the video streaming classes to implement a VideoStreamingPlatform interface and use one #add().

	interface Video{
	public void play();
	}

	interface VideoStreamingPlatform{
	public Video search(String searchQuery);
	}

	class Netflix implements VideoStreamingPlatform{
	public Video search(String searchQuery){
	//.. implement search function
	}
	}

	class Hulu implements VideoStreamingPlatform{}{
	public Video search(String searchQuery){
	//.. implement search function
	}
	}

	class AmazonVideo implements VideoStreamingPlatform{}{
	public Video search(String searchQuery){
	//.. implement search function
	}
	}

	class Hbo implements VideoStreamingPlatform{}{
	public Video search(String searchQuery){
	//.. implement search function
	}
	}

	class CentralVideoStreamingPlatform{
	CentralVideoStreamingPlatform(){}

	public void addPlatform(VideoStreamingPlatform streamingPlatform){
	platforms.add(streamingPlatform);
	}

	public Video searchVideo(String searchQuery){
	//.. implement search function
	}

	public void play(Video selectedVideo){
	selectedVideo.play();
	}

	private ArrayList<VideoStreamingPlatform> platforms = new ArrayList<VideoStreamingPlatform>();
	}

view raw ocp_example2.java hosted with ❤ by GitHub

In Conclusion

The open closed principle is a great principle to keep in mind when writing classes.

How to Diagram Your Cloud Architecture

Cloud architecture diagrams provide critical visibility into the resources in your environment and how they’re connected. In our latest eBook, AWS Solution Architects Jason Mimick and James Wenzel walk through best practices on how to build effective and professional diagrams.

Download the Free eBook