An object should be reduced as much as possible knowledge of other objects. An object need to rely on other objects, but should not be understood that the realization of other objects. An object is dependent on other objects, but should not be exposed to their realization.
Any object should not directly control the generation and maintenance of other objects, but simply references between object-oriented interface reference. All objects carry out their duties, the right to control the overall situation of abstraction.
Day trip there often do not exist for the Yao Jie is not dead. Somewhere own road.
Dependency injection to do is this, Inversion of Control.