Declaring an extension in Jenkins
As we have seen so far, creating an interface or an abstract class is simple once we understand the logic behind them. It's easier to declare an interface or an abstract class, and then implement the required functionality.
Creating a Singleton is also straightforward once you understand when to use each design pattern and which approach suits your requirements.
If we keep this model in mind when creating or adding components to a Jenkins plugin, we should be able to identify appropriate opportunities where it would be helpful to expose an interface and create an extension point for others to use. For example, if you are working on a plugin that for some reason transforms the history of a Jenkins job in to a CSV file so that it can be exported and analyzed in a spreadsheet, you will be writing functions to turn some data in to CSV values—this could be declared as an extension point, and as long as the data passed is of the specified type, others can reuse...
