What is Dependency Injection & Polymorphism

" Dependency injection is the way to decouple conventional dependency relationships between objects. "

What is meant by Dependency ?

Suppose there are two classes A and B. A can not carry out works without B and A can not be reused without re using B. For this kind of scenario we said " A depends on B "

A – Dependant

B – Dependency

What Is meant by Decouple ?

A and B classes are decoupled if A can be use without B and and B can be used without A.

Lets assume creating a drawing application with Square , Triangle and Application classes. Square and Triangle class has same method name called draw() which draws square for a Square object and triangle for Triangle object.

Polymorphism to rescue !

The Greek meaning of the word "polymorphism" is "having different forms". Same object having different object behaviors ! 

Added interface called Shape and the common method draw() for both Triangle and Square. Square and Triangular classes implements Shape interface.

Shape triangular = new Triangular();

triangular.draw();                                                  

Shape square = new Square();

square.draw();

Lets Inject Dependencies !

Created a new class called DrawingTool and added a private member of Shape type (shape) and a setter for that. Created a method called drawShape(). Note that in DrawingTool class does not initiate any object. DrawingTool assumes that It will be provided a initiated object. 

Advantage !

By using this method we can separate dependency from a whole class (DrawingTool) which does not know what to draw. Lets say if we want to draw a triangle we don't have to modify the DrawingTool class. Only thing to do is pass a triangular to the setter. 

Dependency to the triangle that the DrawingTool class has injected by the Application class. This is the way of injecting dependency.