In order to have an application that is easy to add additional features to while ensuring backward compatibility, this application should be respecting the OCP design principle which states that "Software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification".
Classes of this application should be loosely coupled to each other, in other words, each class should depend on interfaces of the other classes not the classes themselves. and that is the essence stated in the Dependency Inversion Priciple.
In this case, any additional feature can be added in a form of new class(es) which will implement the interface of the class(old feature) it can replace dynamically(in run time).
If a given class depends on an interface of a group of classes, dependency of this class and one of the implemntation of that interface should be injected.
Dependency Injection can be achived via a setter or via a constructor and since this operation represents a technical aspect of the application. It should be delegated to a framework, as part of the Inversion Of Control design pattern.
If we use Java, dependency injection can be done by Spring Framework using either XML file or using Spring Annotations.
In this project, In order to deeply understand dependency injection, we demonstrated how can we perform dependency injection statically, dynamically and finally using spring framework. for each method we inject via a setter and via a constructor.