Class Inheritance is defined statically at compile-time and is straightforward to use. It also makes it easier to modify the implementation being reused. When a subclass overrides some but not all operations, it can affect the operations it inherits as well.
But Class Inheritance has its disadvantages too:
- You can't change implementations inherited at run-time.
- Parent classes often define at least part of their subclasses' physical representation. Because inheritance exposes a subclass to details of its parent's implementation, it's often said that "Inheritance breaks encapsulation", as implementation of a subclass becomes so bound up with the implementation of its parent class that any change in the parent's implementation will force the subclass to change.
Object Composition is defined dynamically at run-time through objects acquiring references to another objects, which requires objects to respect each others' interfaces, which in turn requires carefully designed interfaces.
Because objects in Object Composition are accessed solely through their interfaces, we don't break encapsulation, there are substantially fewer implementation dependencies. Object composition helps you keep each class encapsulated and focused on one task. YOur classes and class hierarchies will remain small.