What is Dependency Injection ?

Dependency Injection and Inversion of Control

Whole idea , we do not need to modify this drawing class , you just to pass correspond object to the Drawing class which is a shape object, Drawing class does not really know what it dependent  on , does not really know what it drawing , the advantage is that, if what it draw changes  we do not have to modify drawing class, we can use drawing class to draw all kind of shapes with out even changing a single line of code, the reason is this, the dependency to the drawing class to a shape object  is not owned by  the drawing class, the drawing class only has a attribute shape type class member variable,

The dependency to a triangle class is actually injected to the drawing class by some different class , so this is the principle of dependency injection.
So the dependency is not hard coded on the class , is actually injected by an entity out side the class this is the whole concept of dependency injection.

Spring will play the role of different class, using xml files.