Question

How to decide when to use object adapter and when to use class adapter?

Problem statement:
To create social networking web site and provide import functionality from facebook, google plus and orkut. I am unable to decide whether to use object adapter or class adapter.

I have had look at Adapter Pattern: Class Adapter vs Object Adapter, but could not understand the essence of the difference.

Answer 1

The main difference:

• Class Adapter uses inheritance and can only wrap a class. It cannot wrap an interface since by definition it must derive from some base class.

• Object Adapter uses composition and can wrap classes or interfaces, or both. It can do this since it contains, as a private, encapsulated member, the class or interface object instance it wraps.

The difference is subtle. Usually the later approach (favoring composition over inheritance) is the preferable as explained in the link which I’ll quote here:

Object-Oriented Programing (OOP) has too well known candidates for the
reuse of functionality: Inheritance (whitebox reuse) and Composition
(blackbox reuse). If you try to reuse code by inheriing from a class
you will make the subclass dependent on the parent class. This makes a
system in many cases unnecessarily complex, less testable and makes
the exchange of functionality at run time unnecessarily hard. As a [Clean Code Developer]
you should follow the Liskov Substitution Principle (LSP) when you
need to decide if inheritance is appropriate.

Composition means that one class uses another. You will further
promote decoupling by defining the interfaces clearly. That will also
give you the advantage that implementations can be easily replaced. So
before you start applying the Liskov Substitution pronciple, think
about the Favour Composition over Inheritance concept and ask
yourselve why you shouldn’t prefer composition right away.

"Because inheritance exposes a subclass to details of its parent’s
implementation, it’s often said that ‘inheritance breaks
encapsulation’". (Gang of Four 1995:19)