Difference between dependency and composition?

Simply put :

Thanks to Marko Topolnik for this...

1. Dependency occurs when one object "is dependent" on another. It can occur with or without a relation between the 2 objects. Actually, one object might not even be knowing that another exists, yet they might be dependent. Example : The Producer-Consumer problem. The producer need not know that the consumer exists, yet it has to do wait() and notify(). So, "NO" , dependency is not a subset of association.

2. Composition : Is a type of association in which the "child" object cannot exist without the parent class. i.e, if the child object exists, then it MUST BE IN THE parent Object and nowhere else.

   EG: A Car(Parent) has Fuel injection system(child). Now, it makes no sense to have a Fuel Injection system outside a car (it will be of no use). i.e, Fuel injection system cannot exist without the car.

3. Aggregation : Here, the child object can exist outside the parent object. A Car has a Driver. The Driver CAN Exist outside the car.

The difference can be seen in the two constructors:

• Dependency: The Address object comes from outside, it's allocated somewhere else. This means that the Address and Employee objects exists separately, and only depend on each other.

• Composition: Here you see that a new Engine is created inside Car. The Engine object is part of the Car. This means that a Car is composed of an Engine.